BIG PROBLEM!!!!!!!!!! Slow playback with Imported media

Dear Sir,

I am facing a big problem with AVid Symphony version 6.0.3.  I started a new project, and whenever I import a footage, the system gets slower, and the playback becomes slower and slower and it does not playback the timeline in real time anymore.  I am not using any effects what so ever,  and no disloves between the clips, it is all pure cut.

What is the problem, and how to solve it?

For your refrence, the type of footages I imported are:

1. 1080p 25FPS QuickTime (Pro Rez)  format 4:2:2 10bit at 220Mbps (file size range from 150MB to 3GB ) recorded by Sound Devices PIX240i recoder
2. 1080p 25FPS QuickTime format 4:2:0 8bit at 35Mbps (each file size ranges from 100 MB to 700MB ) recorded by Sony F3
3. 720p 25FPS QuickTime format 4:2:0 8bit at 35Mbps (each file size ranges from 60MB to 450MB) recorded by Sony F3

Awaiting for your fst reply ..

Kind regards