Sometimes I can see that the performance drops for my websites that are located on a VPS. Is it possible the VPS provider allocated less memory/cpu power to my VPS? So, How I can check I get the maximum performance power from service provider?
Normally VPS can be oversold by the provider. It is difficult to do such in Xen VPS. Otherwise go for VPS with a good provider or try XEN VPS with any provider.
Cheap provider oversell it [even if they market the resources as "yours"].
AgA is correct - cheaper VPS oversell their capacity. That is why it is cheaper.
But also, many VPS offer 'burst' CPU allocations - you are guaranteed 1GHz perhaps but can burst to 2GHz. At quiet times, you may be frequently bursting at the higher speed, making performance look OK, but at busy times, you are merely restricted to you minimums.
Also, the less quoted factor is disk throughput. No point in having oodles of RAM/CPU if the disks are get thrashed by too many different users.
Or perhaps your host is REALLY overselling and you are simply not getting your guaranteed quota of resources?
In VM wares, there exist a swap, ie, the RAM of VPS that does not use full RAM are allocated to VPS that needs it, in the form of a swap. This is a good thing for people who need it but your actual RAM may be reduced if you are not using them.
You may use a monitoring software that reads and records the available memory and used memory at specific timings to see if your host is not providing you the purchased resources.