So as most of you know I run a site with relatively high traffic / concurrent amount of connections.
Actually I even hit over 1200 concurrent connections last week and still do so very frequently.
While my front page (Ghost) performs just fine even with one process, NodeBB causes me problems.
Very often the server returns a 500 error, simply because the load is too high.
RAM does not seems to be an issue at all compared to the CPU.
Even though I run 4 processes of NodeBB it still returns these errors.
I currently use NodeJS 5.1. My database runs on Redis.
Additionally I use NGINX to serve static files and proxy NodeBB.
Do you use plugins? If yes, list them.
I assume, Ghost performs less calculations/operations than NodeBB.
P.S.
You didn't specify NodeBB version 
@Nicolas my bad. I use 0.9.1 on 0.9.x (stable) branch.
Plugins:
To be honest the problem already existed in previous releases without some of these plugins.
I adjusted some of my NGINX rules now though and it seems to be slightly better.
@AtonB117 basically they should be common enough. However that still wasn't the actual solution of the problem.
I resolved the failure of additional caching, done by PageSpeed.
