There was a decrease in the
performance of some athletes. (to be honest, "decrease" sounds a bit academical or formal, but then, it´s not wrong at all.)
Here you can see how they use more informal ways:
Read more at http://www.fourfourtwo.com/au/news/bitt ... m1WMvWC.99
...he was unable to perform to his ability – leading to the veteran's release.
...He had a great gesture saying he wasn't being the player he could be
right now," Bittencourt said.