The answer is likely the hard drives, although could be anything. The problem with hard drives are that they are slow, even though we recommend the SAS 15k drives, which are much, much faster than the best desktop drive. But when you’re trying to service 50-100 PCs at the same time, they aren’t fast enough. So what do you do? The recommended answer is to use many SAS drives with a very fast RAID 5 controller. How many – probably 6-20 using a single RAID controller. By spreading the read/write I/O requests are spread out over these drives lessening their individual failings.
Of course, SSD arrays are being introduced that are blazing fast and will take over, or at least a hybrid SSD/SATA/SAS combo will provide the best of both worlds, super high speed at the front, and slow permanent storage at the back end.
Other than the obvious, slow CPUs RAM may actually be the problem as well. Once you starting using very high density RAM, they tend to get slower as the emphasis is on size and not speed. Speaking of CPUs, when you spec out a new rack server make sure that you at least get 2 CPU sockets and 4 if you think that you’ll put 50+ users on the system. You can always install 6 core processors, which when multiplied by 4 socketed chips gives the server 26 CPU cores. You can even get 4 with 12 cores, or 48 CPUs! We’ve only worked with 4×6, which is very, very fast.
So keep in mind about finding where the weak links are. They are a moving target, which means when you change one spec. it will often impact another.