It not the 8 speed cassette what caused the chain drag, since M900 XTR is 8 speed), it is the move to a compact drive front (the modern deore chainset) and then not removing enough links. given it's using a power link (so also a new chain ?) that should be easy to fix by removing a few links.
See if your mate still has any of the old parts lying around that have been replaced![]()
ahhh, makes sense!
My mate got it from another mate who just had the bike and no spares
