Ok, so literary every time that any of my chars enters combat on bg - frame rate drops to 1-5, on lowest settings, while running only one wow acc, on a PC that is able to run at least one WoW on ultra settings (pre 6.0, 15 accounts running smoothly on "low" with good FPS) - tested it across 4 different PC's, and over 10 different chars, it happens only while HB is running, without HB I'm able to play on Ultra without any issues with 25+ fps during the fight.
Now it seems to be happening only during fight in bg's, there's also that weird black square constantly poping up during bg's for "few milliseconds".
I cannot be the only one having that issue, so... did anyone managed to fix it / also noticed it with adjusting some HB / WoW settings?