Does this help?
https://www.thewindowsclub.com/web-page-is-slowing-down-your-browser-firefox
A web page is slowing down your browser.
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Jul 29, 2019
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#3
You have the analysis set to show 5 lives which uses a lot of computing power. I would try to reduce that to see if you still have the same issue.
Its kind of like pressing the gas pedal to the floor in your car and then telling it to not use so much gas 
@ terRorschach Thanks, will check this. Although this is a chess.com problem.
@ jdcannon I doubt this is the reason. It also happens during games.
And if it is a problem to have 5 lines, why does chess.com set it to 5 lines?
jdcannon wrote:
You have the analysis set to show 5 lives which uses a lot of computing power. I would try to reduce that to see if you still have the same issue.
Its kind of like pressing the gas pedal to the floor in your car and then telling it to not use so much gas
Uhm... no.
5 lines consume the same CPU power like a single line, it only takes 5x longer to do the job.
Only on chess.com