SeaBIOS is not slow - it is smaller than filo (both compressed and uncompressed) and loads and initializes faster than filo. During one of my tests, SeaBIOS was 1.5 seconds faster to the boot menu than filo.
I didn't mean SeaBIOS is slow. I meant loading it from disk would be slower than loading it from Flash. But from other replies I see there is not much of a use case for that.