Perhaps if your using them for programming or gaming but do any designers use PC's to run indesign type software, I thought MAC were central to serving this community.
Yes, lot of stuff on the desktop of a Mac can affect performance. Its beyond me about SDD working on Macs, book a genius bar appointment in Churchill square (may have to wait 4 days) or speak to the Mac Man up in Hanover, they will both diagnose and make recommendations I would imagine.