Lots of things could be the cause. Here’s a great article you might like to check out about Optimizing WordPress.
Try using the WP Super Cache plugin, that could help.
As far as hosting goes, that could contribute. If you’re on a shared server (which you probably are), there could be a couple hundred websites all sharing that one server. That means if your site is on a fully loaded server and there are some relatively popular websites hosted on that server, yours (and everyone else’s) could work a little slower.
Doesn’t matter which host you are with, that’s just what happens and is the price to pay for inexpensive hosting.
Plugins could also be the cause. Some plugins are written poorly (coding). Two people could write the a program to do the same thing and one which is coded intelligently could work 10 times faster than the other guy’s code.
Your theme could be the cause. Whoever wrote the theme may have coded it poorly.
In essence, there are a ton of reasons why your site could be working slow. If it works fast sometimes and then slow other times, I’d say it’s hosting. Probably not the fault of your host, it’s just a fact of life with shared hosting. You could purchase a Virtual or Dedicated server, but that will cost you bucks and take a lot more work to host.
