I do not know why Hangouts load and show messages and SMS very slowly and it take long time to load it.for example I must wait for load one message or SMS
I've noticed the same thing lately. very annoying, especially when some of your responses are fairly important. you end up sending 2 or 3 before you realize the first hadn't sent and now you don't know what order they will go in. im sure Google will resolve it soon enough.