Very slow to start processes or open applications

I have a Mac Mini (Mid 2011, 8GB RAM) with Yosemite which is getting slower and slower. But the weird thing is that only opening applications or starting new processes (from the terminal) is terribly slow.

Once an application is started (it can take up to 5 minutes to open Excel) then it works without any problem.

What I looked so far:

• the log files seem normal


• monitoring the system with opensnoop shows a lot of activity from Spotlight (mds_stores ist the process using more CPU time and mdworker)


• in general the CPU is almost idle


• disk seems healthy (no S.M.A.R.T. errors, no problems while checking it with Disk Utility)

Where could I look for a hint on what is making the machine slow?