Game has unplayable frame rates!

[imacken]

OK, thanks for that. I'm using the GOG version, not Steam.
However, you are right about the texture quality. I turned it down to Medium, and everything is OK now.
Strange, as I have everything else switched on and set to max. With Medium Textures selected, everything is good, but the minute I switch on High, I'm getting 1-2 fps, even if I turn everything else down to a minimum.
Other system stuff - 16Gb RAM, Samsung 850 SSD