It is impossible to answer the question without having full information.
It is unclear what kind of sites kind of load, whether they are cached, what kind of load they generate on the CPU and disk.
It is likely that the drive is very slow - rather than suffer the majority of VPS can and the CPU time is simply not enough, and maybe the bandwidth to the Internet is not enough.
Where VDS need to watch to understand what to do?
First you need to watch how much and what kind of resources you need to tap everything.
And then see what resources are not enough.
Well, still need to understand what virtual servers are in most cases going with oversells.
And the neighbor on the physical server running on your VM hard task may well dramatically reduce the performance of your VM.