Hi, There is more than one way to say this, of course, but I would personally say, 저는 문자하는 속도가 느려요 or "my texting speed is slow". Hope that helps. :)

I don't think I can do that one specifically, and I'm not 100% sure if this is natural but an alternative could be "I send messages really late" is (저는/나는) 문자를 늦게 보내(요). Sorry, that it's not the exact one you wanted!