Only few plug ins

HI everyone,
I’m having some difficulties to manage plug ins in Ardour. It can manage just few.
In my actual session just 6. Once I add the 6th Ardour begins running odd:
I can listen to my session just once and a weird sound is added on top of my actual one, like the Hard Disk is not fast enough so I have to relaunch it to listen to my song just once and so on.

I the message list in JACK there’s a long list of this:

JackEngine::XRun: client = ardour was not run: state = 1 JackAudioDriver::ProcessAsync Process error

My session has 10 tracks and 2busses. All the plug ins are in a bus.
My external disc (where my session is) is a 7200RPM via USB

USB consumes allot of CPU cycles. You should move your session to an internal drive. If you have a USB interface make sure that it is not sharing and IRQ with the drive.