I have Summoning skill capped but I'll agree it's really slow to skill up. I have the earring from Gustav tunnel so it was slightly more bearable to cap. Best bet is to use skill up food, do some GoV till you get a few levels of magic/melee skill up prowess and or skill up when martial master is around. Even with all of these things combined the rate is crap, but you'll most likely cap smn magic long before SE does anything, or says they can't because of X and Y.
I somehow capped smn skill and yet can't seem to cap enhancing skill, even on SCH which has D base... I've tried spamming spells on my lvl 95 fellow and still can't seem to cap. So yeah, magic skill ups suck.