Currently there is no option to cancel a current operation in Picard, the only way is to quit Picard altogether, i.e. abort, kill process. Sometime I can accidently click on a wrong folder and load 100+ of files and Picard can start loading info for matched albums and recordings and that's 1000 of requests. Or there can be a network error and requests get stuck and never finish (well, it will finish on timeout by it's like 5 mins wait per request, but for 1000s of requests it will take forever). While quitting the app works as a last resort, you will lose all the the entered data, tag edits, album version, etc that were not yet saved, having a "Cancel" button would be much better option.