Sorry, a bit off topic but,
What's_nitro?, Damn your signature. When something says don't and you can click on it, you forget about the n't part
Back on topic:
How hot did the ESC get the last time you ran it? If it (or the negative wire) was hotter than normal, the car being slower is probably something to do with that, rather than anything else.
If you are confident soldering (and/or the warrenty has run out), then try resoldering the -ve wire back onto the board, or if you are less confident with a soldering iron (like me), and know anyone who races 1/10 scale stuff, ask them, as they probably will have had to deal with solder tabs at some point.