The Minimig-AGA core has been developed since 2014 at least, so it's strange that performance hasn't improved. Maybe all the focus has been on improved compatibility.
What kind of performance improvements can you expect from something that is intended to be as equal to original as it can? I honestly cannot tell the AGA on MiST apart from "real" AGA, except from the one well known horizontal shift bug on low screen modes. I understand from Chaos, that going from ECS to AGA was not really that much work (which echoes what I recall CBM engineers also have written before about AGA... a quick and cheap upgrade of the ECS chipset)
Improving on AGA is what SAGA is about.
