Check whether work happens in the browser. Most tools on this site process input locally and do not send the payload to our servers.
JSON format, JWT decode, regex, and timestamps are the best offline fits: short input, instant output.
Image compress and QR can also stay local. If a tool asks to upload a file, confirm it really needs the network.
Open the JSON and JWT tools in the dev category and retry offline. If format and decode still work after a refresh, the page is doing the compute.