Largest Contentful Paint is usually the metric that tanks a Core Web Vitals report, and on most sites the LCP element is a hero image — a JPEG or PNG shipped at 3-5x the byte size it needs to be. The fix is boring and well-documented: convert to WebP. What's less documented is how to actually do that for a whole folder of images without installing a CLI tool, paying for a cloud API, or uploading your assets to a server you don't control. Here's how our own JPG to WebP converter does it, and what's actually happening in your browser when you use it.
What's actually running when you convert an image
There's no File System Access API and no Web Worker involved — I want to correct that here since an earlier version of this post overstated the tool's architecture. What the converter actually does is simpler and still fully local: for each file, it draws the image onto an in-memory <canvas> element with ctx.drawImage(), then calls canvas.toDataURL("image/webp", quality) to re-encode it. That single browser API call is doing the real work — it's the same WebP encoder Chrome and Edge ship internally, exposed directly to JavaScript. Nothing leaves your machine because nothing needs a server to run: the encoder is already sitting in the browser you're using to read this page.
Files are processed one at a time in sequence, not in parallel across worker threads. For a batch of a few dozen images this is fast enough that you won't notice — canvas re-encoding a typical hero image takes somewhere in the tens of milliseconds. If you're converting hundreds of files at once, expect it to take proportionally longer rather than instant, since it's not fanning out across CPU cores.
Step by step
1. Find your actual LCP element first
Don't guess. Open Chrome DevTools → Lighthouse (or run a PageSpeed Insights check) and look at the specific element flagged as your Largest Contentful Paint candidate. Converting images that aren't on the critical rendering path won't move your CWV score — converting the one hero/banner image that is will.
2. Drop your files into the converter
Go to the JPG to WebP tool (or PNG to WebP if that's your source format) and drag your files into the drop zone, or click to select them. Conversion starts immediately per file — there's no upload step to wait on because there's no upload.
3. Set quality before you convert the batch
The quality slider (1-100) is applied to the whole batch, not per file, so set it before dropping your files in. See the settings guidance below for what value to actually pick.
4. Download and check the size delta
Each converted file shows its before/after size with a percentage change next to it, so you can see the real savings per image rather than trusting a marketing claim. Click 'Download all' to get every converted file, or download individual files as you go.
5. Serve it with a fallback
WebP has near-universal browser support in 2026, but the safe pattern is still a <picture> element with a WebP <source> and your original format as the <img> fallback, so nothing breaks on an edge-case browser or an image viewer that fetches the raw file.
What quality setting to actually use
85 is a reasonable default and what the tool's quality slider starts at. For a photographic hero image, 80-85 is where most people can't distinguish the WebP output from the source in a side-by-side comparison, while still cutting file size meaningfully versus an unoptimized JPEG. Push it down toward 70 and you'll start to see visible artifacting around high-contrast edges. For flat-color assets like logos or icons converted from PNG, keep quality high (95-100) — WebP's lossy encoder handles photographic gradients well but can introduce visible banding on flat color fields at lower settings.
A mistake worth flagging
Converting your LCP image to WebP doesn't help if the <img> tag rendering it is still missing width/height attributes or a fetchpriority="high" hint, or if it's buried behind a render-blocking script. Format conversion addresses one variable — file size — not the others that also feed into LCP timing. Check the full picture in DevTools before assuming a format swap alone fixed the score.
Ready to see the byte savings on your own images? Use the Batch JPG to WebP Tool — everything above happens in the tab you're already in.