455 is not a problem at all with that kind of server - if it's dedicated. I've done more with less. First of all, the ST column number indicates it's likey a virtual machine - steal time is almost always 0 on a dedicated machine, or much, much lower. Might be due to another VM on the box.
What are the specs, so far as PHP version goes, if opcache is turned on, and database stats? Do you have addons that might have an excessive query count for the forum home (as well as not properly indexed queries)?