CPU performance on newer Intel, W/ e-cores

[Whitling2k]

Now then!  I appreciate this is an old game, but I occasionally pick it up and play a few Tormented Space maps.

I recently upgraded my PC from an Intel i7 6700K (4+4 core), to an i5 14500 (6+6 performance and 8 efficient cores).  Same everything else (GTX 1080, 16GB DDR4, SSD etc).

I had no bother running CW3 on my old CPU, with some crazy large and busy maps with 100s of units.  On my newer CPU, my framerate starts to drop after something like 100 units. 

Any advice?