When using fast forward on the gba im lucky to even get to 70 fps. I have banana on another sd card and it gets between 500 and 700 fps depending on the game. I have made sure all of my retroarch settings are the same, using the same core(mgba) and governor(performance).
SOLVED!:
I replaced the core that came with pixie(0.11-dev 0f39405ec) with the version that was with banana (0.11-dev b2564482c) and FF started working 
Just to double check, are you 100% sure the Banana device is using mGBA? gpSP was the default in Banana IIRC, and it’s waaay faster than mGBA. I didn’t think mGBA could even get up to 700fps on these devices.
Yes, i changed to mgba because gpsp wasnt activating retroachievements. I even double checked when verifying fps on banana.
Which indont really care about equaling it exactly but right not fast forward does nothing and ive tried everything i can think of. I would be happy woth even 2 to 3x
I’ve having the same issue with mgba, do you mind me asking how you changed the core version?