DISCLAIMER: since Cannabis is a banned substance and illegal in most countries/states, it should not be used recreationally or for sporting performance. This is just an interesting look into Cannabis and sporting/exercise performance.
Recently, a systematic review by Kennedy,  was performed looking at Cannabis (AKA marijuana, weed) and exercise/sporting performance. Cannabis has a long history in sport as a pain relief ointment at the classic Olympics (as cited in). During the 19th century, Cannabis preparations had a widespread use in Western medicine for varying ailments such as sedation, convulsions, asthma, appetite stimulation and muscle relaxation. However, newer and more effective and more easily administered medications have been developed. Also, the fact there are many different compositions of Cannabis makes dosing unreliable (as cited in).
THC is the cannabinoid that induces psychological effects. These acute psychological effects depend on the dose, how it is administered and experience of the user. These can include alterations in mood, relaxation and higher functions of perception and impairment of reaction time learning and memory. Adverse effects can occur after single large doses such as anxiety, panic attacks and hallucinations . So can Cannabis enhance exercise or sporting performance?
What The Review Found
Original research by Steadward et al.  investigated THC administered through pipe inhalation on sub maximal bike exercise and grip strength. THC caused peak work capacity to decrease but no change in grip strength was observed. No subjects could detect having taken THC or placebo. Further studies showed increases in blood pressure and heart rate with sub maximal exercise as well as decreases in maximal work duration due to leg fatigue in maximal capacity tests.
Data is very scarce in regards to Cannabis and exercise performance. There are no studies currently looking at strength or athletic performance and no papers have specifically measured sporting performances such as running times, weights lifted or accuracy in a target related event . Internet sites generally recommend sportspeople to use Cannabis for its relaxing properties rather than increasing strength or speed . An example would be for asthma as THC is a bronchodilator meaning it opens the small tubes in the lungs wider. However, there are more effective medications at present that don’t have psychological reactions. There are claims that these psychological effects may have a calming effect before events but this has not been researched in a clinical trial . There is also no data to support the muscle relaxing properties of Cannabis.
So far, based on the current research we can conclude that THC does not enhance performance in aerobic exercise while research is still needed in all other areas of exercise/sporting performance.