Pingdom Tools can be good to determine what part of a web page loads slow.
Second some web hosts limit the number of concurrent users to between 50 and 300 depending on server package. Others do not.
Most of the ones that don't, will have other limits in place that essentially do the same thing - ensure that the chances of a user taking up the whole server for himself are pretty slim.
As for your VPS, there will be no such limit, or if there is one, you can modify it. However, you still have no guarantee that your VPS has enough computing power or memory to handle traffic peaks. For example, with a 512MB VPS, you only really have 200-300 MB to actually power your site, the rest is used up by all the services you must run to have a working server in place.