I don't know if this pertains to your SO, but my co-workers are my second family. I spend 40 + hours with them a week and I have a good relationship with them. I would love for my SO to meet them and interact with them on occasion. With that being said, I wouldn't make my SO get all fancied up to impress them. I think that this situation is running a little deeper-- that maybe he's making it seem like it's about them but could this possibly just be something he wants you to do to show him that you care about him? You mentioned you took too long and he got upset, could it be because he feels like you won't make that effort for him? Your boyfriend has met your co-workers many times, so he does that for you, regardless if your co-workers are more sociable than his. Now he may feel its your turn, and sometimes we just need to suck up these little things and do it to show your interest in his work environment regardless on whether you really are interested or not.

Also, how do you know that they are judging you? You are judging them as well by assuming they are judging you. Everyone judges everybody, its nature. Maybe they didn't know what to say to you and it was easier for them to all converse amongst each other because they know each other. Maybe they are socially awkward. Maybe after a few more interactions both parties (you and them) would open up. You said only the stock guy said hello to you. Did you say hello to them?

It is give and take in a relationship. In this situation it seems like its important to your boyfriend that you make the effort, and I think this is something small enough you can manage. Yes, he should accept you for you-- no doubt about it, but if it'll keep him happy to slap on some lip gloss and throw on your dress pants instead of the jeans on occasion and you feel he is worth it than just do it.

Don't worry about what they think or know about you guys. Good luck.