What you should do to increase performance depends on what you use the VPS for, and where the bottleneck is.
If your site rely on a large amount of database queries, switching to SDD's can help performance. If your sites are mainly static content, a SSD won't help you much.
If you are using all of your available RAM, and resorting to swap file usage, increasing your RAM will help.
If you have a slow port speed that is not able to keep up with the traffic, increasing your port speed will help.
Then you must consider the software side of things. What web server are you using? Litespeed and nginx will give you better performance then apache.
Is you database optimised?
Is your site connecting to the database using local host or 127.0.0.1, or is it having to resolve the IP via DNS (MySQL.domain.com)
Is your website properly optimised?
Do you properly utilise gzip compression and caching.
Etc, etc, etc.