Are your BW 1+2 ROMs AP-patched?This post is just me wanting to give as much info as I possibly can, I don't know if this is something that can be fixed on either end but extra data always helps.
I use a UK/EUR DSi that I recently put hiyaCFW and made DSiMenu++ the replacement menu
this was and still is on unlaunch 3.0
I've used all regions (apart from J) Pokemon roms and the freezing stays the same, White 1 + 2, Black 1 + 2 seem to freeze for me in trainer battles with the battle music continuing. And I'll call them 'delays' when travelling to another area/building.
HG + SS seem to be fine apart from random freezes and apparent rare loading bug where the next area is just not there followed by a crash.
Also, there is no Unlaunch 3.0.