John, I can't comment on your specific setup although it seems well able to cope.
I have PlayIt Live running on an old laptop (i3) and it takes about 10% CPU. With the other stuff I (and Windows) have running the system is at 30% typically. The hard disk is solid state but the system is nothing special and will play a cart without issue.
All my audio is CBR MP3.
I would wonder about the sound cards. First thing is the sound card set to the same sample rate as the files ?
Your cards seem to both be PCI, Also Amazon reviews for the Delta 66 suggest possible driver issues. Also it may be that two M-Audio cards in the same system is the issue.
I guess the cart is on a different sound card to the main audio.Try the carts on the built in soundcard on the PC as an experiment.
Personally I would have one sound card only if possible (the ESI Gigaport HD+ USB card works well on the i3 laptop).
I wrote up (some of) my sound card experiences at
https://support.playitsoftware.com/support/discussions/topics/5000081440
I've had quite a serious sound card "habit" and if you would like a few more reviews I might do some more.
Although I don't currently have it installed, I really liked the Hercules DJ Trim 4&6 USB Audio interface and the driver support is good. there is a 2 in 6 out mode which is reasonable for PlayIt Live.
It's not a current device (the market is now really only DJ controllers which are not suitable for PlayIt Live) but you might find one on eBay.I
t's built like a tank !
John Vee