I am convinced that cannabis improves concentration and eliminates feelings of boredom. I always was average at best at football. I smoked cannabis before a match once. It was by far the best match I ever played and I scored thrice.
Of course, this is anecdotal evidence and could be a coincidence, but I am firmly convinced of the effect.
It's always tricky extrapolating from focal cognitive effects to overall performance. For example, small amounts of cannabis or alcohol can also greatly attenuate the obsessive internal rumination (a.k.a., "the jitters") that can sabotage many kinds of performance.
Of course, it's dose-dependent, because eventually the detrimental effects on attention and memory (as you correctly point out) will outweigh the beneficial anxiolytic effects.
> It actually impairs attention and affects short-term memory.
And it kills your sperm as well. I think that these are long-term effects though, I was talking about the direct effects (the dose probably matters as well).
It actually makes sense that the use of cannabis improves attention but impairs attention in the long run. Compare it with drinking coffee. The first time I drank a strong cup, I had a strong boost of energy and felt a little euphoric even. I am now a heavy caffeine user, and when I don't drink coffee I actually feel more tired than usual.
Sorry, but after 10 years of smoking almost every day (and stopping for months completely in between), I can assure you that this isn't true. At least not in the generalized form. Everyone is different. I have ADHD, for example. Cannabis naturally calms me down to actually focus, it makes me function. I start cleaning and programming where 10 minutes before I was all over the place mentally. I've tried Amphetamines but the side effects are just incredible. Of course there's also the guy who will let his apartment go to complete waste and stop caring, but you can't generalize effects of a substance. Some react completely differently.
I was also stating that cannabis improves focus for me. That is, directly after use. I have never been a regular cannabis user so I can't comment on my experience on the long term (I was just stating that I wouldn't be surprised if cannabis had a negative effect on your focus in the long run - just like prolonged caffeine use has on your energy level).
Of course, this is anecdotal evidence and could be a coincidence, but I am firmly convinced of the effect.