That's a knows timing issue.
The "name zooming" effect in the end credit sequence of StarFox is extremely heavy on the GSU (SuperFX) chip (yes the real one too) so the timing goes off by a few seconds over 3+ minutes of the end credits.
Same thing with Snes9x btw, it is not perfect either.
In fact Snes9x has the end credits lag MORE than on the SD2SNES.
Perfect timing is VERY hard to reach with emulation or on FPGA.
What Kevtris did with the SuperNT is far beyond simple, and does not apply to the special chips implementation of the SD2SNES.