SNES carts used ROMs made in two speeds: SlowROM (200ns) and FastROM (120ns). I can't remember if those were official names or fan names.
It's not important to the player, just how the games were programmed.
(yes, SlowROM games could be hacked to FastROM to speed up gameplay, but that is a whole different topic that's probably already been done somewhere)
A "checksum bad" means that the checksum is bad. If it's a retail game, it might be badly dumped.
If it's a prototype or a hacked game, then it's probably nothing to worry about.
Prototypes usually didn't bother calculating the correct checksum, until final code.
And in hacks/translations, the authors usually didn't bother to fix the checksum (though a few did, as a means to tell the player if the game was patched correctly).