The commonest causes of slowdown are:
Added software that means more startup tasks, so things running in the background taking memory and CPU time.
Disk filling - the more data added after a format and install, the more the head has to travel to cover the used area.
Disk fragmentation - files having data scattered around the drive rather than in consecutive sectors.
Registry bloat - many programs use the registry for random data storage & the bigger it is, the slower it is to access.
Running ccleaner to delete leftover temporary files, then in its tools - startup section, disabling all non essential stuff, then finally running its registry cleaner, can restore some performance.
Then get "mydefrag" and run that, using the system-monthly option on the C drive.
A tool for disk defragmentation and Windows optimization. Fast, low overhead, with many optimization strategies. MyDefrag (formerly JkDefrag) is able to handle USB sticks/disks and floppies....
www.majorgeeks.com
It will take a long time, but it is exceptionally good at optimising the drive for fast access.
The two together can often get a machine back to near-new performance.
(You can use the ccleaner part on any machine, but do not run a defragger on a solid-state drive, it will do no good and can shorten the drive life).
There are quite a few possible reasons for apparently similar systems running at different speeds. The most obvious are memory speed, memory access mode (single/dual channel), CPU bus clock, CPU cache size and hard drive speed / cache size.