My personal musings on the subject. I worked as a lab tech for some time and it seems to me that servers do have physical/electronic limits. A server running at Xmhz speed will require X amount of time to scan its memory. More memory usage will slow down the system. The memory has a physical limit, which can be expanded but it is finite. A better hosting company invests more money in hardware, more memory, faster processors, to serve it's clients.