I must confess I a not the best for engine diagnostics
, and I do not know how the decompressor functions on the XT, but it sounds to me like this is due to the decompressor cable being badly adjusted.
Did you know the cable was loose or was this a guess. I mean did you compare play with manual recommended freeplay and adjust it accordingly?
I am speculating, but I would start by backing of the decompressor cable so that there is a bit more free play, so you know it is not affecting the valves when the lever is not being used.... See if things improve
If at all possible, I would do a compression test, now and once you have adjusted the cable again: see if there is a difference in readings and if they are both within tolreances...
If I'm way off, I'm sure XT Xperts will be along to sort you out soon. Meanwhile, enjoy the rest!!
HTH & good luck
Adventure: it's an experience, not a style!
(so ride what you like, but ride it somewhere new!)