Basically there appears to be a fuse on your microcontroller that somehow got corrupted. This fuse causes your microcontroller to run 8x slower.
As a long term solution, you'll need an ISP programmer (between $16 and $36) to fix the fuse. Its very easy to fix.
(or it could be a mistake in your code somewhere, but less likely)
Its not actually a big deal so no need to panic, but if you feel you need more processing power in the future, let us know and we can help.