I got my Hifiberry DAC+ ADC Pro yesterday, and bought it for low its very low latency. I installed raspberry pi os, got Jack up and running and started Jamulus. Works well so far.
I then connected a loopback-cable and started jack_delay to measure the roundtrip-latency. According to two independent reports, cited by Hifiberry on their blog, latency should be around 3.1-3.5ms at 48KHz, 64 frames, 2 periods.
However, with those settings I consistently get a latency of 5.05ms roundtrip.
That is still very good, just not as good as I expected. I tried lots of settings and none made a significant difference at 48KHz. At 192KHz I could get insanely low latencies, but I really need 48KHz, 64 frames.
This is raspberry pi OS, which means a 5.4 kernel, no PREEMPT support and an older Jack version. I will try Ubuntu with a preemptive kernel once I receive my micro-HDMI cable. That might help, but I'm not sure.
Any ideas on how to lower this further?