I have reinstalled GTA V to test this out. Once again, I only ran this test once. The only issue I noticed was that only objects at the bottom half of the screen were being rendered in, until I got closer to the more distant objects. This looks like graphics kind of popping into view. There may be a term for this that I'm currently not remembering. RDR2 does not do this, not in game nor in benchmark. The GTA benchmark displayed FPS during the test, which RDR2 did not. So it's hard to tell where and when RDR2 suffered FPS losses. I thought GTA V may have suffered from more vehicles and people, but it seemed fine when it was well populated. It suffered more when it was not, oddly enough, so I think something else may have been going on to cause that. Now, as for the results, GTA V's benchmark test revealed a minimum FPS of about 18, average about 54, and max about 74. I don't recall whether or not VSync was turned on, though I have it enabled globally within my Nvidia control panel settings. The only settings I changed were my resolution and to DirectX 11. GeForce Experience seems to want you to run a game and then exit so that it can change to optimal settings, so I exited the game. Then it said my settings were optimized. I should note, though, that I believe I have GE set up to automatically optimize games for me.
If this is true, I find it to be very impressive! It's great when a great looking game runs smoothly on your PC. It seems Rockstar found a post-GTA V way to further optimize the RAGE engine used. I read some of a really long article (https://www.eurogamer.net/articles/digitalfoundry-2019-what-does-it-take-to-run-red-dead-redemption-2-at-60fps) that seems to kind of support this idea, at least some what. Some of the points I've made are there in that article. Keep in mind, too, that I'm running a GTX 1660 Ti, an i7-8700, w/ 16GB of RAM, and on an SSD. I'm not exactly pushing resolution too hard, but I'm really impressed w/ what I've experienced so far (about 100 minutes).
I also want to point out (though not really relative to FPS), as for size, GTA V is about an 86GB download while RDR2 is over 100GB!
Anyway, let me know if you have any experience w/ this or if you've seen something I haven't. I think I may look around on the Internet more to find out what I suspect. I'll also have to do my own testing. I'll try to add to this post with the results that I come up w/, especially if anyone's interested in this discussion. *Update added post edited with GTA V test results*