It's not generally the act of soldering the pipes, but the fact that you turned the water off then back on. This can dislodge built-up crud in the lines. remove the aerator of the faucet and clean it out. That usually solves the problem. While you have that off, run the water full-blast through it for a bit. Some of the restriction will be in the aerator, so removing it will flush more crud out when you turn the water back on.