Does cannabis affect cycling performance?
Are you asking about the immediate effects while high or long-term effects?

I can't find any unbiased, well-cited sources except for one from the British Journal of Sports Medicine which basically says that doctors should try to keep athletes from using cannabis due to its possible dangers. However, some sources (of unknown quality) claim that it may help with some sports due to e.g. its ability to help people get into a pseudo-meditative state (which assumedly helps with e.g. long-distance running). Others claim that its ability to lessen the perception of pain may be of benefit in some sports (e.g. in combat sports).

However, cannabis use has been more or less uncontroversially proven to slow down nervous-system response (c.f. "Cannabis use and cognitive dysfunction") and so it can be dangerous to use when reflexes are necessary for one's safety (i.e. such as when riding a bicycle). Moreover, smoking cannabis is still smoking --- the long-term health consequences of which are hard to overstate.