From the journal Sports Medicine, in 2003:
Studies have demonstrated that the use of cannabinoids can reduce anxiety, but it does not have ergogenic potential in sports activities. An increase in heart rate and blood pressure, decline of cardiac output and reduced psychomotor activity are some of the pharmacological effects of THC that will determine a decrease in athletic performance.
This was found with a quick search, but note that there are probably other, better papers relevant to this.
In response to the comments, and working just from research literature, it seems that there's no clear answer. The majority of the research focuses either on detecting drug abuse or examining drug abuse prevention programs rather than studying how it affects performance. This paper discusses some of the problems around drug abuse, and includes some interesting comments about individual drugs. However, the general feeling can be summarized by a line in the abstract of this article:
Potential health complications represent a serious risk to an otherwise healthy population.
Basically, yeah, you may increase your performance, but you're almost certainly going to damage to your body, possibly quite seriously, in the process. I know, this isn't quite earth-shattering, but this is common sense. Marijuana is a drug, and drugs are not good for you. Smoking marijuana will have all the typical side effects involved with smoking any substance, including the potential for lung damage (and the decrease in blood oxygenation that results) and the increased cancer risks. Additionally, long-term marijuana use is associated with numerous mental health risks. True, these are not specifically related to exercise, but they should be considered before using drugs to improve athletic performance.