Well, good if sound is working. The snd-hda-macbookpro-dkms-git pkg could have been installed with pamac GUI (if you have the AUR enabled) or via:
pamac build snd-hda-macbookpro-dkms-git
it was meant as a smoother and more recent alternative to the ‘solution’ of the davidjo/github driver. Still, that doesn’t explain the slowdown.
Why this happened I don’t know; I’ve put manjaro on old 2008/9 mbp/imac and a 2017 macbook air and I now always start off by installing xfce minimal. I avoid KDE because it brought the graphics to its knees when later the system gets more decked out and the eye-candy options are used.
For the same reason I’ve never tried Gnome/wayland on these machines but I wonder if it’s the best option for your Intel Iris Plus, especially since it could handle xfce easily. If this is indeed the cause of the slowdown or why your pulse-audio icon doesn’t show I don’t know but on xfce this should be a speedy machine for regular tasks.
To get the icon do a search in the gnome section here, I think this might help: Volume keys and volume icon not working - #2 by axelinux .