A solid performance is a performance where there is nothing to complain about; it is "completely good with no bad parts," to quote the dictionary. The opposite would be a weak performance, where nothing was done right. Other close antonyms are flimsy, flaky, ramshackle, and slipshod, which all describe something that is poorly built and will fall down in a light breeze! Solid is used to describe things that are well done and well put together, because, well, wouldn't you rather live in a solid house than one made of paper?

If you give a solid performance, it was very good, though not great. Likewise, if you achieve a solid win, you win by a large, but not huge, majority.