Garageband error message "Disk too slow etc"

This disk too slow message appears almostevery time I use GB. Never had this issue running my old Pro-Tools 9, a much more powerful S/W on Windows 7.

Im just playing 1 x MP3 backing tracking track, and 1 x Guitar i/p with Fast Track USB I/F while not playing the instrument!

Hardware Details:-

GB 10.1, Elcap 10.11.3

Mac Mini Late 2012

16GB DDR3 RAM

1TB HD (with a ton of spare space)

System info (while running GB)

CPU 85% idle

RAM used 5.23GB (of 16GB)

I have tried all the usual suggestions, i.e. Disabling the Fast Track device, Track Lock, Uncheck 24Bit, Running GB without other programs.

I have just now tried re-aranging the order of the tracks i.e. Guitar No1 and MP3 No 2 and so far seems OK but this is not a satisfactory solution more of a quirk!

I have run far more complex mixes with previous OS X builds.

I'm considering upgrading to Logic but no confidence at the moment

So my message to apple engineers, Please respond to this query