What Buffer-size are you using? (Ardour > Menu > Window > Audio/MIDI Setup)?
With USB devices try at least 256 (@ 48KHz), most systems require special tuning to go below that.
You cannot. The soundcard is used for I/O only.
It is not possible to outsource arbitrary DSP. Some soundcards have built-in effects (for direct monitoring), some have programmable gate-arrays, but this only works with a fixed set of usually proprietary effects.
I seem to have had the wrong idea about sound cards. I remember Pro Tools or some other massive DAW I used a few years ago going out of its way to make sure I had a sound card before launching so I assumed they provided some acceleration.
Pro Tools used to be bundled with audio hardware made by avid. “Bundled” in the meaning of “license is paid with the outrageous price for the hardware” so Pro Tools just won’t run without the audio card it came with. Which you probably lost and totally forgot about it or left at your aunt’s studio or your dog ate…