As a project becomes more complex with extra characters and animations, you will see a natural slowdown of the FPS. This happens on all systems, however you are using quite an old graphic card (about 7 years old) so this will also limit the frame rate you can get.
Try loading the sample project '11 G3 Animals' from the Project > Demo folder and make a note of the average frame rate you get. For comparison with my Nvidia GTX 1080 I get a steady 52fps throughout.
Also as the GTX 760 only has 2GB of onboard vram, try lowering your 'Max Real-time Map Size' in Preferences (CTRL+P) to say 1024 x 1024 to see if this improves performance. Also keep 'Real-time Vector Render Quality" on the Low setting while testing performance.