
Thanks for your support so far.
I made some more tests and found out the following:
If I created a simple new test map with reflective floor and an additive light over it, the GT315 DOES WELL with 2.2pre-1876. (No matter whether gl3 option is switched on or off)
If 2.2pre-1876 is run WITH gl3 option on the GTX750 DOES slow down on my existing maps (not on the new test map).
Now, if I remove all things from the existing map and thereafter just add one single additive light above the reflective water, the GTX750 slows down again.
Without the gl3 option the map works fine.
Can it be the case, the map is too complex? The felective area is large, there are several linedefs around it, and alltogether it has around 1700 sectors.
Does it make sense to send you the map without customised resources? (The file would be several MB when adding the required resources to it.)
Suggestion: Wouldn't it be a simple solution to let reflectiveness be switched off via the options menu in GZDoom?

Once again: Big thanks, Mr. Graf!
zhd