I have a Quantum, which I run with Logic Pro X 10.4.1 on a 2017 iMac 27" i7. macOS 10.13.4. It's a single-thread monster, and didn't quite expect to be overloading a single core at 44.1k/32 samples with fairly modest CPU loading - as in an amp sim, a compressor, and some basic room reverb. My previous (class-compliant USB) interface never even got close to overloading with this kind of a channel strip set-up with the same buffer size. Granted, RTL was like 5.6 ms with that interface at 44.1/32... not 2.0ms as reported by Logic with the Quantum.
Granted, this isn't exactly a dealbreaker, since 64 and 128 sample buffers are perfectly usable most of the time... it's just that I have some instruments/external effects that incur their own latency when I play through them, and it helps a lot to minimize overall RTL as much as possible with those.
I guess I just expected that the CPU loads would be identical for the same buffer sizes. And I expected that 32 samples would be mostly usable at all samples rates, with the Quantum - this has turned out to not be the case... 32 samples is effectively unusable for me.
Has anybody else seen this problem?