Audio problems during first playback

I have a project started with C5.5 containing about 60 tracks (40/20 audio/midi).

I loaded the project into C6. Everything works ok, but somehow C6 is slower than C5. When the song is played back first time after loading C6, every track produces annoying crackles and pops when audio is introduced on that track. Also, I get hd overload warnings. Once the project is played back from start to finish, things get back to normal.

This did not happen in C5. I have not changed my system in anyway.

Anyone experienced something similar?