I don't quite understand how FastCGI works or whether I really need it. It seems to have helped on shared hosting. I'm seriously thinking of moving from shared hosting to cloud hosting. Will I still need FastCGI?
You need it or not is totally depend on what kind of website you hosting right now. But since your current shared host still can cope,why shift to cloud hosting?
With my membership site, page loading times are occasionally long, i.e. slow. I think I'd be alright if I had a small site but traffic can get heavy with my membership site (text mainly; no heavy downloads, videos, images, etc.). So I thought I'd make the switch to cloud hosting to deal with both the speed and traffic.
Perhaps just an overloaded host? How much traffic is considered 'heavy' in this case? Dynamic/static site? Depending on certain factors, cloud hosting might not be what would help you, you might just need to let your host know about the issue or to find a new one if they won't fix it.