Can't pin point the problem, but I experienced pretty much the same with my old mac mini. It's probably a combination of factors: HDD gets fragmented over the t

Can't pin point the problem, but I experienced pretty much the same with my old mac mini. It's probably a combination of factors: HDD gets fragmented over the time, new software and versions of OS are more demanding, drivers not optimized for old hardware, etc. Some of this can be solved by upgrading to SSD or clean OS reinstall, but not all.