The target heart rate zone for exercising is generally between 60-80% increase of your resting heart rate. Most people rarely hit this when weight lifting unless lifting weights that are too heavy you for you, with little to no rest between sets, and with extreme intensity. Marijuana on average only increases your heart rate about 30% on average (obviously varying between individuals) and the effects of marijuana on the heart diminish rather quickly. So smoking before a weight lifting set could actually benefit your work out by the increase in heart rate. I wouldn't suggest smoking and running a triathlon but to say you shouldn't smoke before working out at all is a rather baseless suggestion. That being said good luck having the motivation to life weights after smoking. 1 or 2 draws off a vape, though, can really get your mind in the right place.