I ran identical sessions on A2 and A3. The sessions included two tracks created with imported audio files. One plugin - the CAPS 2x2 Plate Reverb - was used on the Master track in each session. The differences are notable:

A3 produced xruns reliably on opening and during operation, including playback. With the test file up to 25 major xruns are typical. The DSP usage idles at ~20%, going to ~30% on playback, with at least one spike to 58%.

A2 produced 0 xruns. DSP usage idled at ~8%, moving to ~10% on playback. No spikes.

Ardour2, 2.8.11, rev 8197
Ardour3, 3.0pre0 rev 8413

JACK 0.120, home-built
  realtime
  default priority
  frames 128
  sr 44100
  periods 2
  timeout 5000
  mididriver seq

Ubuntu 10.04, 32-bit
kernel: 2.6.31-11-rt #154-Ubuntu SMP PREEMPT RT

AMD64 3800+, single-core 2.4GHz CPU
4G memory

M-Audio Delta 66 audio interface

I will furnish the session files on request.

Dave Phillips
3 January 2011