[Jackaudio] Soundcraft ui24r latency

Hello, I've been gigging for years with a carefully fine-tuned manjaro box where I run jack with 4ms "internal" latency (-r 48000 -p 64 -n 3). With my Presonus Audiobox 1818VSL, that leads to a roundtrip latency of ~5.7ms (as measured by jack_iodelay), which I'm very happy with. Now tried replacing my 1818VSL with a Soundcraft ui24r, same jack settings. The roundtrip latency jumps to a rather unimpressive 9.9ms! So the "added latency" is almost 3.5 bigger with the Soundcraft. That's a pity, because this really is a nice machine otherwise... Is it expected to find such enormous differences between interfaces - or did I just forget the -Xmagical parameter that will fix it all? Any idea welcome, I'd love so much to love this gear! ;-) Thanks, Matthieu