Why the Gamebuino META defaults to 25FPS

Hum can be interesting to have someone making some assembly coding part. If i don't make mistake it's what some do one Arduboy to be able to optimize some parts of games. The goal of the Meta is to let every body be able to do his own games uickly and have fun but it's not a reason than some others could not make their own code for specific games and maybe that their improvments could be later add in specific libraries or in the current if it's can be interfaced easy to let everyone use it. 

Yeah, the main thing is that the GBC used separate chips for graphics, sound etc. and the META has to CPU-drive all of that. That overhead isn't to be underestimated.

Isn't the game boy also doing everything from the CPU chip? A custom hybrid between the Intel 8080 and the Zilog Z80 which is called Sharp LR35902.

The z80-like CPU only does the game calculations. Tilemapping etc. is done by written to magic addresses, which is where the graphics chip takes over. Same goes for sound.

Yes indeed, I checked it, I was confused because both the GPU and APU are actually physically integrated in the CPU integrated circuit.