Thanks @DollyDaydream, this sounds identical to the issue I was having, which turned out to be due to Kaspersky’s free security cloud (I’m not sure if this applies to other AVs). It might have been possible to solve the issue by tweaking the settings since I’m guessing that it was something to do with the active monitoring by security cloud in the web browser which probably could be selectively switched off somehow, but I just ended up uninstalling it (which solved the issue).