In sync mode I got:
172.273 frames 3.589 ms total roundtrip latency
extra loopback latency: 44 frames
use 22 for the backend arguments -I and -O
It looks like the position of -S is important, as once the parameters start describing the ALSA audio config, it is then interpreted whether to use 16 or 32 bit samples.
exec /usr/bin/jackd -S -t 2000 -R -P 95 -d alsa -d hw:pisound -r 48000 -p 64 -n 2 -X seq -s -S
As I understand, the reported ‘extra loopback latency’ is the time spent in the codec, this value is consistent with the expected time (~1ms, this includes both input and output processing). The rest of the time is spent in software buffers.
Btw, I wasn’t aware of the Sync flag before, it does report better latency, is it a good idea to have it on by default, or are there some possible caveats?