After a while the browser starts to lag

[Admini]

Yep, deleting site history (58k) and download history (5k) solved the problem
Heh

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Maybe we need to automatically delete history that is more than 2 months old?

Chromium only keeps History for 3 months by default.
Older History will be deleted automatically.
Maybe the file size is not real reason, it is possible that this file gets partially corrupted.
I think there is a bug in Chromium that can easily corrupts History database.
And what anti-virus are you using?