No realtime playback at all

hi,
iam working on a 8-core 3ghz macpro with 16 gbyte ram.
i do not get any realtime-preview although i enabled caching.
working in syntheyes or aftereffects is nice but mocha is loading (caching) everytime i start a project but the playback is not realtime.
even a sequence with only 50 frames (1920x1080) is not playing near realtime.
what settin did i miss?
anybody a solution?
thank you all