Client-side vs server-side compression

Server-side is the conventional model. Your file is transmitted to a server, processed there with whatever tooling the operator runs, held for some period, and sent back. This is how most well-known compressors work.

Client-side processes the image inside the page using APIs already built into your browser. The file is read into memory on your own device, worked on there, and handed back as a download. No network request carries the image, because there is nowhere for it to go.

Being fair about this matters, because client-side is not universally superior.

• Heavy codecs. A server can run mozjpeg, pngquant or a slow AVIF encoder with settings that would be impractical in a browser tab.
• Very large files. Browsers are limited by device memory. A server is not.
• Automation. An API you can call from a build pipeline is a genuinely different product.
• Weak devices. Processing on a server means the work does not happen on a five-year-old phone.

• Privacy. The file never leaves the device, so there is no third-party processor, no retention policy to trust and no breach that can expose it.
• Speed on small batches. No upload and no download means no waiting on the network at all.
• Cost. No servers means no per-image economics, which is why client-side tools tend not to meter usage.
• Working offline. Once the page has loaded, it keeps working without a connection. Nothing server-based can do this.

For a holiday snap, it does not. For a good deal of professional work, it does.

If you handle client material under an NDA, patient images, identity documents, personnel files, legal evidence or unreleased product photography, uploading to a third party is often the thing you are specifically not allowed to do. The tool being free and convenient does not change the obligation.

The useful question is not "is this tool trustworthy" but "does this tool need to be trusted at all". Client-side processing removes the question rather than answering it.

Open your browser's developer tools, switch to the Network tab, and compress something. If your file appears in an outgoing request, it was uploaded. If it does not, it was not.

A stronger test: load the page, disconnect from the internet entirely, then compress an image. A client-side tool keeps working. A server-based one cannot.