The 7750 isn't exactly slow, and Rust has some very modest requirements. The best place to start, would be to run some tests and verify the system health and then check the load while gaming.
To start, my usual recommendation is to get HW Monitor, CPU-Z, Furmark and Prime 95. Install and start both HW Monitor, and CPU-Z, keep both where you can see them, and still work. Then install both Prime 95 and Furmark. Start Prime95 first, use "small" or "in place" FFT's. Then start Furmark at an 800x600 window. Watch HW Monitor for excessive temperatures (over 70 C per core CPU, or 90C on the video card) and watch CPU-Z for throttling (slower than the "base" speed). If all goes well, move on to step two.
At this stage, you can safely remove the above software if desired.
Then download MSI Afterburner. Use this to more closely watch your GPU temps, and GPU load. Leave it running, hop into your game, and when it starts to lag, alt+tab out to Afterburner. See what your GPU load looks like.
Please report back with the results.
Laptop: Alienware 14 : Intel i7 4700mq : 8GB ram : Nvidia GTX 765 : 256GB Plextor M3 : 1080P IPS display
Test rig: AMD Phenom X4 955 @ 4.0Ghz : MSI 970A-G46 : 8GB Ram : 128GB Plextor M5s : 2x AMD 5770's (Flashed to 6770) : PC Power and Cooling Silencer 750 : Pioneer BR
Hackintosh : Gigabyte GA-H61m : Intel Celeron @ 3Ghz : 8GB ram : EVGA GTX 550Ti : Patriot Torx 2 64GB : Silverstone Strider ES-50 : OSX Mavericks