Kinsta, WP Engine, SiteGround: What Their Built-In Image Optimization Actually Does

Many managed WordPress hosts advertise “built-in image optimization” somewhere on their pricing or feature pages. It sounds like one feature, but in practice, each host has taken a different approach, and each one solves a slightly different part of the image optimization problem.

We went through the actual documentation for three popular managed WordPress hosts, Kinsta, WP Engine, and SiteGround, to see what’s actually included, how it works, and where each approach naturally has boundaries. Here’s what we found.

Why “built-in” can mean different things

There are two different approaches behind the phrase “built-in image optimization.”

One is CDN-level optimization: when a visitor requests an image, the CDN checks whether their browser supports WebP and serves an optimized copy on the fly if one is available. The original file in your media library stays untouched. Kinsta and WP Engine both use this approach, and both are powered by the same underlying service, Cloudflare Polish.

The other is plugin-level optimization, where a WordPress plugin compresses the actual files in your uploads folder, the same way a dedicated image optimizer would. SiteGround’s Speed Optimizer takes this approach.

Each has its own advantages. CDN-level optimization typically requires little ongoing setup or maintenance once it is enabled, and never touches your original files, which is useful if you want to keep your source images untouched. Plugin-level optimization changes the files themselves, so the results carry over everywhere those files are used, not just on requests that happen to pass through a specific CDN.

Kinsta: CDN-based optimization with lossless and lossy modes

According to Kinsta’s Image Optimization documentation, the feature is included free for every Managed WordPress Hosting customer and is available when Kinsta’s CDN is enabled, which is the default for new sites. Once enabled, optimized copies are generated at the CDN edge, so they don’t count toward your Kinsta disk storage.

You’ll find the setting under Sites > CDN > Image Optimization, with three options: None, Lossless, and Lossy.

• Lossless creates WebP versions of GIF and PNG images with no quality loss, and strips metadata from all images.
• Lossy does the same, and also converts JPEG images to WebP, using compression that can reduce quality.

The WebP version is only served when a visitor’s browser supports it and the WebP file is meaningfully smaller than the original.

Kinsta’s approach offers a simpler set of controls than a dedicated plugin: you choose Lossless or Lossy for the whole site, with fewer options around formats, exclusions, thumbnails, and bulk processing. AVIF isn’t part of the current feature set. For sites that mainly need a fast, no-maintenance baseline, that trade-off is usually a reasonable one.

WP Engine: automatic, lossless CDN optimization

WP Engine explains in its WebP Image Optimization documentation that its image optimization runs through Cloudflare Polish. On sites using Advanced Network or Global Edge Security, Polish is enabled by default and image optimization runs automatically once the site is using that network configuration.

There’s one limitation worth knowing about: WP Engine runs Polish in lossless mode, which converts only PNG and GIF images to WebP. JPEG images keep their original format, they get their metadata stripped and a lossless recompression pass, but no conversion to WebP.

Because a lot of the photography on the web (product shots, blog headers, portfolio images) is saved as JPEG, this means the built-in tool offers more limited gains for JPEG-heavy sites specifically, even though it works well for PNGs, GIFs, and general assets like icons or screenshots.

WP Engine also lists third-party image optimization plugins, including ShortPixel, in its own performance and image-optimization content for users who want additional control over compression and formats. WP Engine additionally offers NitroPack as a paid add-on that bundles image compression with other front-end optimizations.

SiteGround: file-based optimization through Speed Optimizer

SiteGround takes a more hands-on approach. As documented in SiteGround’s Speed Optimizer image optimization guide, Speed Optimizer (formerly SG Optimizer) is a free WordPress plugin, and its image compression actually rewrites the files in your media library rather than serving alternate versions from a CDN. Under Speed Optimizer > Media, you can:

• Choose between multiple compression levels
• Preview and compare the result before committing
• Back up your original images automatically before they’re compressed
• Turn on WebP conversion separately

This gives site owners more direct control over how the files themselves are processed than a CDN-based approach, and the backups add a safety net the other two hosts don’t need, since they never modify the originals in the first place.

There are a couple of boundaries worth knowing. PNG files larger than 1MB aren’t currently supported by the compression feature, and AVIF isn’t available as an output format. It’s also worth noting that while most of Speed Optimizer’s other features now work on any host, the image compression specifically is exclusive to sites hosted on SiteGround.

Side-by-side: what you get by default

Feature	Kinsta	WP Engine	SiteGround
How it works	Cloudflare Polish, at the CDN edge	Cloudflare Polish, at the CDN edge	Speed Optimizer plugin, rewrites actual files
Cost	Free, all customers	Included with Advanced Network; GES is a paid extension	Free plugin; compression runs on SiteGround servers
JPEG → WebP	Yes, in Lossy mode	Not converted lossless mode only	Yes
PNG/GIF → WebP	Yes	Yes	Yes (PNGs over 1MB excluded)
AVIF	Not yet	Not yet	Not yet
Compression control	Lossless or Lossy toggle	On/off only	Multiple preset compression levels
Touches your actual media library files	No	No	Yes, with backups
Adaptive/responsive images per device	Not included	Not included	Not included
What happens after a host migration	CDN-generated versions no longer apply	CDN-generated versions no longer apply	Already-compressed files remain

Where the built-in tools have different boundaries

A few areas show up as boundaries across all three approaches, not just one host:

• AVIF isn’t available yet. AVIF typically compresses further than WebP at a similar quality, and none of the three built-in tools currently offer it as an output format.
• No adaptive, per-device resizing. None of the three provides adaptive, per-device image resizing as part of the built-in image optimization workflow, SiteGround’s Maximum Image Width setting caps upload size site-wide, but that’s a fixed limit, not resizing based on the visitor’s screen.
• The built-in tools expose a relatively small set of site-wide controls. Dedicated optimizers can add broader options around formats, exclusions, thumbnails, and bulk processing.

These aren’t necessarily shortcomings; they reflect the narrower scope of built-in optimization compared with a dedicated image workflow.

Where a dedicated image optimizer adds more flexibility

Built-in optimization is a strong convenience feature, and for some sites it may be all that’s needed. The difference with a dedicated optimizer is that image compression itself is the product, rather than one feature inside a larger hosting stack.

ShortPixel has developed its own compression algorithms around three optimization levels, Lossy, Glossy, and Lossless, which offer the best compression on the market. Rather than applying the same treatment to every file, ShortPixel tests different ways of compressing each image and aims for the smallest result that still meets the quality level you’ve chosen. Lossy prioritizes maximum size reduction while keeping a good visual balance, Glossy preserves more detail for quality-sensitive images, and Lossless keeps the result pixel-for-pixel identical to the original.

That goes beyond a simple site-wide WebP toggle or a small set of compression presets. ShortPixel Image Optimizer can bulk-optimize an existing Media Library, automatically process new uploads and selected thumbnail sizes, generate both WebP and AVIF versions and deliver all images via the ShortPixel CDN.

Just as importantly, the optimization workflow isn’t tied to the hosting provider. The same compression engine and settings can stay with the site if you move from Kinsta to WP Engine, SiteGround, or somewhere else.

That doesn’t make the hosts’ built-in tools less useful. They prioritize convenience and require very little management. ShortPixel simply goes deeper into the image optimization itself, with better optimization, more control over compression, formats, thumbnails, and how the Media Library is processed.

The bottom line

There isn’t one universally better approach here. Built-in optimization emphasizes convenience, while a dedicated optimizer offers better compression, better quality, more flexibility and control. For most sites, the right mix comes down to how much of that flexibility you actually need.

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