Hey hey, that's really unexpected. There could be various reasons, also on client side as the 2.5Gbit drivers are still not perfect. Especially the igp driver has some power issues, did you try to switch off EEE ? Further on OPNsense side there are many options to optimize, see the docs some hints might be "activate PowerD", "TSO" ... Could you also share your current OPNsense version ?