Some of what has been said might not be the answers you want to hear, but all of the replies I've seen so far are definitely relevant to your question. Nobody here is judging you, they are showing concern for your well being and what could potentially happen if you start using meth at work. To answer your question, yes, of course meth will increase your work performance - by your own admission, you've done meth quite a few times so you should have a reasonable understanding of its effects.
However, you said you've never used at work, and the problem here is that some people who begin using to increase performance, energy, motivation, productivity, etc. find they can't work properly without the drug. If you can limit your use at work to a few times a week and still maintain an otherwise normal lifestyle, then go right ahead, we're just trying to warn you that it can be a slippery slope and you should keep things in check if you're going to start doing so.
For what it's worth, there are plenty of other stimulants that I'd recommend using at work before meth, but you seem dead set on it, so what's stopping you?
