I have 3 accounts set up in Kmail (2 gmail + 1 yahoo), with automatic mail checking every 15 minutes (same for all). Whenever the checking happens, all my cores could be 100% for quite some time. I've tried changing my.cnf, setting thread concurrency to 1, assuming perhaps my MySQL config was the culprit. However, I can see 3-5 (2 mail filter agents if I'm not mistaken, the other 3 is the email account) threads in kmail, which makes me think perhaps the devil is not MySQL itself. Is it possible to limit mail checking to single thread, doing it for one account each time rather than concurrently for all? I'm using Manjaro with self installed KDE, with latest 4.14 KDE. Not using Plasma 5, so it's still pure KDE4. Just ask if you need me to give output of certain commands or config file or anything else.

OK, currently I solve "the problem" by turning off "download all messages for offline use". At least it drops the huge network traffic to a much smaller one.