HwKeeper
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While I was sending the previous message and everything was freezing, I looked in the browser's Process Manager and the hw was eating more than a gig of memory, not counting video, screenshot, alas, I didn't take one. I don't know, probably for a modern engine that's normal. So I'll be like a cuckold, just watching from the sidelines what's going on here.
This is clearly something abnormal, we need to look into it. I installed the same theme, also measured a single tab, only 133 MB and no CPU load.
I'll keep looking for what's eating the memory.