I won't be the one to stand up and defend Rift's performance, but I will say that one of the big differences between Rift and the other two games is that Rift uses a licensed engine. As I understand it, the big holdup keeping Rift from getting real multi-core support is that there's no-one at Trion who knows the engine well enough to re-write the relevant parts, where both ArenaNet and Blizzard had the capital to build good engines from the start.
The irony is that Carbine managed to pull off building a phenomenal engine from scratch for Wildstar (which runs night and day better than Rift), even though it was their first production. Read into that however you want