the game's files will be loaded from wherever they are installed to. If installed to your mechanical drive, load times will be slower than when installed to the ssd. how much slower depends on the ssd.
But once they're loaded to the ram/video card, performance should be identical no matter where theyre installed.
So if you can tolerate waiting a few seconds longer, and need the extra space, install the games to the mechanical drive.
One thing to ensure is that the windows page file is set up on the ssd and not the mechanical drive. If windows is installed to your ssd, you shouldn't need to do anything.
As jbseven said, the system will read game files from where they're stored. If you're looking to use the ssd as a cache for the games, you can look into things like Intel Smart Response Technology (Not to be confused with Rapid Storage Technology, the raid driver and such) or if you have enough Ram you can try putting the game on a RAMDISK.