I didn't bother to read thru every post so if what I'm about to post has been covered in another post, sorry for repeating it. If some know-it-all, stuck up asshole, of which this site has plenty, has that big of a problem with it, fuck 'em.
To OP/any future reader that may find this helpful : I would imagine the safest "speed" would be the green vein Kratom, especially some ultra enhanced whatever variety. All stims/speed drugs (which I can't stand, my name is downer for a reason. It doesn't mean I haven't done my share of cain, MDA/MDMA, tons of acid, various designer shit, etc...) are rough on your body to various extents and different speeds can be harder on different parts of your body. Most are toxic to some degree (cain, for example, is extremely rough on your ticker, ecstasy causes nerve damage among other things, so I hear, etc...) They are basically a rough-on-your-body class of drugs that commonly have among the list of lovely side effects anxiety, paranoia, agitation, heart palpitations, delusions, the ever-so-lovely stimulant induced psychosis, restlessness, insomnia, the list goes on. But, if you're set on doing them, I would think Kratom, being a relative of the coffee tree and with a long history of use with minimal side effects, would be safest. The green vein strains are the speediest kind (I think Meng-DA is supposed to be pretty "up" without feeling like you're on crack). I know that my personal bias plays into my opinions of speed drugs, but the side effects and medical info are from medical sources I've read (and I don't mean Wikipedia, I mean real ones), as well as many speed and crack/powder cain users I have known that were full-on into that kind of stuff. Prolonged use (or a heavy dose) of even what are supposed to be mild to moderate speed (like Dexedrine) can make you a hyped up, crazy mess acting like a rat on acid. I've seen it and would need high dose opioids and benzos just to keep from shooting a few of these people (even then, after eating high doses of oxycodone or doing large shots of heroin, plus eating double digit mg doses of Xanax with booze, I have come close to popping a round in some of these people).
I would definitely try it before doing crystal meth, that's among the most toxic drugs they got. I moved to the mountains of the rural South about 3 years or so ago now and crystal is a big epidemic here (back home it was heroin and crack that were the main hard drugs, along with plenty of pills of whatever pharmaceuticals you want). I've seen what it does to people and I would not want crystal in my, or anyone I care about lives. If you need a booster for the Kratom, I would try white crosses or some kind of trucker pills like that. At least they're OTC at lots of gas stations and head shops, and both they and Kratom are legal so you have the advantage of not worrying about getting arrested. I feel like I need a heavy dose of oxy and xanny just thinking about speed to write this.