Slowness may be determined by many factors, definitely not because you went to your frind, and came back, it must be a mere coincidence. The first thought is that your disk may be full, how much free disk space is there? (right click on the disk, then Info or command-I). The second possibility is that you installed something incompatible with your system. A thrid cause may be a dying disk. And some other possibilities.
You must have at least 10 % free disk space for a good run.