Unlike in native pc games, what affects your fps in Dolphin the most is the CPU. What the GPU does is simply allow for more advanced effects and higher resolutions to be shown. It's kind of hard to explain here, so check out the Dolphin FAQ page for more in depth info: (https://dolphin-emu.org/docs/faq/)
So no, a 750 Ti doesn't guarantee no lag because that's all up to the CPU, but 1080p should be more than doable. A new graphics card will not improve the current emulation speed, but it will make the game look better.
So no, a 750 Ti doesn't guarantee no lag because that's all up to the CPU, but 1080p should be more than doable. A new graphics card will not improve the current emulation speed, but it will make the game look better.