Latency down to 34ms with soft mode and "no memory lock" — jackd raspberry pi

So I am trying out a Raspberry Pi 3B+ as a headphones protector, running a cheap USB sound card in duplex mode and sound processing being done in Rakarrack. The sound system hanged a lot with "xrun" error messages, even with very conservative sizes of buffer (lots) and sampling frequency (low).

By selecting "no memory lock" those hangs disappeared and were replaced with clicking sounds when the runs happened.

I then ticked "soft mode" and the clicks disappeared. I can now run with low latency, currently 34ms. Since this is for headphone minitoring I don't mind if samples get dropped; it's not for recording.

Maybe I should try to put memory lock back… Anyway, great performance progress!

Update 2019-03-07 I put memory lock back, seems to work fine, so it was the soft mode that made the difference.