It’s directly related to the USB DAC. I think that’s obvious.
squeezelite or any other app is not working on such a low system level. BUT. Everything is possible. I’m not a designer (just an advanced user).
My wild unsubstantiated guess would be something is potentially wrong on the USB stack
(XMOS<>RPI firmware/drivers). I do know there have been issues/challenges on the isynchronous handshake in the past.
Most important. Other people should have a look and confirm this behavior.
Then you, as manufacturer should have a closer, look at it.
There are indications that the high IRQ rate could in fact be related to the isynchronous USB mode.
The XMOS, as clock master, is basically causing all these interrupts.
The big Q is if the actual absolute IRQ amount is OK.
Or if a certain DAC firmware setting could lower that IRQ load.