Without knowing anything about your system and its graphics hardware, comp settings, what exactly you use with RT3D and so on nobody can tell you anything.
Ok, you are right, thanks Mylenium.
Here are a few details about my system :
iMac, 27-inch, Late 2009.
Processor 2.66 GHz Intel Core i5
Memory 4 GB 1067 MHz DDR3
Graphics ATI Radeon HD 4850 512 MB
And about After effect stuff
I am working in 1280 X 720 (half HD) comp size 25fps
I want to convert simple text into 3D text for logo animation.
I hope these inputs are helpful to suggest me something.
I see you only have 4 gigs of memory. AE likes lots of memory. If you stuff your machine with RAM, you'll like your AE experience more.