It possible Manjaro is the culprit. Is that the only OS you are using?
EDIT: Culprit meaning that the Manjaro driver for On-Board Intel video cards does not play well with Unity.
Viewing post in Can't start Linux version in Manjaro
Hi! Sorry I wasn't clear before. The first machine I tried (the one with the old Intel graphics) was running Manjaro Linux with kernel 4.14. The other one I tried (with 20GB RAM) where the entire system slows down is running RedHat Enterprise Linux with kernel version 3.10. So no Manjaro is not the only Linux I'm running...