Very slow spotlight indexing

On a MacMini OSX.6.8 Server (244 GB used on 500 GBdisk) spotlight searches produced bad results, i.e. a lot of non-found files which under normal conditions must be found.

I decided to relaunch de indexing with

sudo mdutil -E /

Now the indexing is executing, but terribly slow. Time to be finished is 1570 hours ( and this after a while of indexing, not at the very beginning) !!!

What need to be done to come back to a normal behaviour.

Thanks

Urs