On my new-ish business class dell laptop CPU goes to 7% when my QA400 (the older unbalanced model) is capturing. But if I switch from 24/48ksps to 24/192ksps and increase the FFT window size to 65535, CPU goes to 20%.
I got an answer to a direct email question from Matt at QuantAsylum yesterday, and he thinks a Minix Z83-4 would do nicely, as would almost any PC with a few cores and 4G or more memory.
And this agrees, come to think if it, with my experience of Reaper with a dozen active plugins on an even tinier PC.
I never connected, but Reaper has plugins that run several fft processes in parallell and in real time at that. And I never had a problem with a Bay Trail cpu at 1 G and 4G memory.
As you can see in that blog entry: one of the z83xx based fanless pc:s with 4GB memory is more than enough.
Only thing that might get to be a problem is logging: you should have some cloud storage or similar available if you intend to keep a lot of logs.
Personally I'd plug in a fast usb stick or a TF memory card for such things.
Tiny keyboard, smallest monitor you can get and a fanless PC plus a QA401 will make a great instrument setup for audio work.
An i5 is old? Define old? I used to run my QA400 on a Toshiba that is literally 10 y/o and it worked fine. Maybe you just need to re-install the OS and clean out the crud. Frequently those machines have a factory image on another partition. You just need to re-run setup.