I'm sure they are very forgiving. I have gone to Catholic masses and Episcoplian ones with friends (they kneel too in traditional churches) before even though I grew up Presbyterian and have been going to Baptist churches since meeting H (we haven't been in ages), and Protestant churches stand for some prayers and most if not all hymns though there is no kneeling. Point is, no one seemed bothered at all if I was slow to sit or stand or kneel with everyone else. And there are elderly people and handicapped people who do sit the entire service, and it's entirely fine.