NO idea. Seriously. The problem is that you sayx nothing abou thte bottleneck. A single core 1.8ghz cpu should handle 20 users without problems.
- We dont know how loaded the host is network wise
- We dont know whether you HAVE a 1.8ghz cpu. It says so, but VPS are shared - could be you just have a 10% time slice overall.
It looks like the VPH is totally overlaoded, and sadly host overloads are not really analyzable from the client. I run some high performance VPS - but on my own hardware, so I always know when the CPU on the host peaks, or the IO falls behind, for example.
I would suggest doing some performance tests, and then poissibly moving to a more reputable host? Linude, for example. It is not VPS per se that are slow, but if I max out a machine, with cheap VPS offerings, then naturally.... well... not every host really has what he sees.
A physical server has plus and negative sides. Plus: you know what you have. Negative: if it fails, you have a problem. Same with updates. It mostly makes sense when you have multiple.