I'm having similar FPS problems with Tekkit. Vanilla Minecraft runs just fine, but Tekkit slows to a CRAWL after about a minute or two. 20 FPS would be great. I'm getting unplayably low framerates (less than 1 FPS, sometimes as few as 1 frame every 10 seconds). The launcher told me Tekkit needed an update, so I updated--and that's when it started.
Intel Core 2 Quad 2.4GHZ
GeForce GTX 460 graphics
250GB HDD
4GB RAM
Java 7u3
Win7 Pro x64
It's an old PC but it still runs pretty good.