I have some discomfort with arms out too, like playing cards or board games, reading books...I can manage it for 1-2 hrs and if i move around every so often.

I use a pillow on my lap to rest the books on
play card games on the computer LOL
only play quick board games and change positions a lot

for me it is the blood getting stopped when my arms are at 90+ degree and also the additional leverage? of hold arms up/out and the change of pressure on the neck/shoulder/back