For friend to friend, it's hard and I do think it depends on age a lot of the time. I'm 29 now so if one of my friends got into a relationship they were really happy with/excited about I'd be happy for them. I have my own life and live with my boyfriend so not hearing from a friend or seeing them as much isn't a big deal at the stage I'm at in my life, as long as we did still plan to catch up and see each other when it suited us both. When I was younger if someone blew off plans for a guy it would be annoying but I've also been on the other side of it when you're in a new relationship and really just wanna see them, so it's hard to judge!
Same as moving too fast really. It's such a personal thing. I'm not particularly emotional so generally take things quite slow haha but a lot of my friends will go on a tinder date for example, it goes well and they are constantly texting, seeing each other every few days etc and that seems so keen to me
Red flags for seeing a friends relationship unfold with their partner I notice a lot more! Any phrases with them saying things like "I'm not allowed to do this" "I better head home now incase X gets worried/angry" "you can't come over because X will be home" etc. Anything where it sounds like they've been told not to do something or see someone would concern me!