Rockstar's done a bang-up job tuning GTA V for PC, but if you're running modest hardware, what graphics settings will net your the biggest framerate gains with the smallest drop in quality? Tweak guides for high-end titles are almost mandatory these days and the internet has provided for the latest game from the Grand Theft Auto franchise.
Should you disable anti-aliasing or shadows? How about anisotropic filtering? Steve Burke at Gamers Nexus has you covered with a rather in-depth guide explaining GTA V's various graphics options and the effect they have on performance. At the top of the list is, unsurprisingly, multisample anti-aliasing, which remains one of the more gruelling operations for GPUs. Fortunately, most will be happy with its faster, if blurrier counterpart, FXAA.
Next on the list is post-process special effects and grass quality. If you don't mind forgoing a bit of fidelity when it comes to smoke, heat shimmering and the like, you can wind the former back to "Very High" or "High", depending on your setup. Using "Normal" for grass quality can also reclaim some performance, though the article mentions it's situational, as city areas will be affected less than open ground.
From here, the benefits of using lower settings aren't so pronounced. Reducing shader quality can result in a nice boost with very little in the way of visual differences and shadows can safely be set to "High" from "Very High" with a negligible impact on prettiness.
If you're still desperate for higher framerates, hit up Gamer Nexus for more potential optimisations, though expect diminishing returns. Maybe have a crack at 4K after your next upgrade.