It seems that Cloudflare increased some kind of proof of work level in their captchas around the start of this week, since every time one shows up it drains all of my CPU for 20 seconds before giving up and looping, or occasionally, after 10 minutes of refreshing and redoing the captcha, letting me through.
It doesn't even have the decency to ask for an interactive challenge after clicking the checkbox, it just wastes CPU cycles... I'm sure the real bots love that.
I dont play silly games.
However, in the past I have noticed that some addons prevent the completion and create a continual loop.
disable each addon if you use them one at a time to see if that makes a difference.
you dont say which OS you are using.
If you are not tech minded install bleachbit and run that to clear out all the hidden rubbish in .cache etc.