I am very scared to change anything to under Appearance (Global Theme, etc) as this may break the performance again 
You have to make sure the themes are updated. A workaround is to change only the color-scheme and Plasma Style.
Thank you @anon89812132, that is exactly how I approached it this time. I did not download any themes but rather just downloaded Plasma Style, Application Style, Colors, Icons, Cursor which is really the same.
Heh, randomly the performance is again down:( I give up
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