Some plugins causing serious buffering issues
 

Not sure if anyone else is having these problem but iZotope RX5 DeClip causes slow playback as well as SoundShifter from Waves.

Mac Pro - Mid 2010
2 x 3.46 GHz 6-Core Intel Xeon
MacOSX Mountain Lion
64GB 1333 MHz DDr3 ECC
NVIDIA GeForce GTX 770 2GB
1TB Western Digital 7200RPM
2 6TB Software RAID Drives set at 0
UAD Apollo Firewire

Also using Quartz Debug for Waveform glitch