The deal is that I took long in one session. So far so good, he criticized me for it. But he keeps doing it. I have all my characters/critters whatever scripted in a spreadsheet, and have spells well sorted out, at least for combat. Round actions take 30 seconds, maybe a minute. He still keeps bitching that I'm taking too long. I keep telling him that I only need to hit a button to do MY fourteen attacks, while he has to roll 6 attacks and a fuckton of damage dice by hand, who will be faster? Doesn't disconcert him in the least. And he's a fairly nice guy, normally, he just got it into his head that I take too long. (The actual reason is that he can't compete game/character wise, so this is really a penis contest in disguise. I've offered help, and he has been partly receptive. Still, there is no complete resolution in sight.)
Now I don't dispute that I (and the GM) DO need to do something about that. We actually did, there is just this remnant of stress remaining, which we haven't managed to get rid of.
To be honest, this fortune-cookie advice 'play a good game' stuff is getting annoying, because people are afraid to give specific advice. So here's what I'd say:
First off, this is not your fault. I don't care if fault is a problem solving vehicle or a jet to Paris, you shouldn't have to solve this problem yourself. Assuming you're not doing anything wrong, the most you should have to do is speak to the DM. Unfortunately, throughout the years, there have been some really rude GMs who have abused their power, so GMs now are afraid to put their feet down, even though they're the ones taking responsibility for the game.
If I were DMing, I would bring a stopwatch to the game, then take an average of your rounds throughout a combat V the Dbag's rounds per combat. Then afterwards, at a break, I'd take the Dbag into the hall and show him those averages, then ask him why he's being a Dbag. Likely, he was just messing with you and didn't realize it was getting to you. Unless the guy says it's jealousy, I doubt it. If he's not taking the game seriously enough to pay attention, he's probably not that concerned about his power level. But it was very nice of you to try to help him with his character. If he's disruptive in general, I might ask him to avoid the caffeine (If I were DM), or ask him if he'd like to take more frequent breaks to alleviate boredom. However, I would still inform him, nicely but firmly, that he had to lay off. That being said, if ribbing makes you nervous (and I'm not faulting you at all for that, I've played with a lot of shy people over the years) I might ask you yourself to lay off the caffeine, or ask if there's some way to make you more relaxed during the game.
Either way, it's not your fault at all. If there is a problem, it is for the problem player and the DM to solve.