Are you sure it was the proxy and not JavaScript in your browser? I've seen similar behaviour from one particular website where using it in a certain way causes it to process a lot of data slowly and block the main thread. This somehow persisted across tabs, even if I closed all the tabs and tried again that site was still hanging until that process finished. But if I used incognito or another browser it would he responsive.

It might be something similar if all of your sites are subdomains. Try incognito at the same time next time