I've found that a lot of manastones on their own don't cause much lag, but when you have adjacent items that utilize the mana stones in some way (powering them up, requiring them, etc) then the calculations get insane and that's when lag happens a lot.
For example, if you have like 10 manastones and a wand, it's not that bad. 10 manastones a wand and a shield ring that uses mana, and now it's churning.