Hello & welcome to the forum!
A good practice is to first provide info about your system:
inxi --admin --verbosity=7 --filter --no-host --width
Also take a look at,
On a sidenote, I first installed Manjaro KDE on a i3, 4GB RAM laptop, and it took about a minute to go to the login screen and ran at fairy normal speed. So Manjaro isn’t slow. 