Wow, I absolutely did not expect this to accumulate this amount of views. In regards to PC specs I am much better than previously, but I still hope this tutorial serves for somebody.
I have kind of forgot about GTA V and games in general; nonetheless, I am still open to suggestions.

Hey guys, I hope this was useful for high-end PC players, though I'm afraid it wasn't. It was more of a tutorial for people who still have something like a 9 year old Dell notebook. It might be useful for people such as DarkViperAU, who have a fuccin RTX 3090, and if it wasn't, I'm sorry. Probably going to update this thread soon.

@InfiniteQuestion Oh I see, I've had this problem in a different way though (when I actually had a GPU). I had this problem at certain points of the map (high places, like the top of the Mount Chiliad. It didn't really bother me and I just forgot about it anyways but it indeed looked strange.

settings.xml is an important file used for storing in-game graphical settings for GTA V. After the release of the game, low-spec PC players took full advantage of this file in order to make the game much more smooth than it originally is at its lowest.
But now, things have changed. On this tutorial, you will learn how to create and customize your own settings.xml file. Read below and try to make a good setting compatible with your PC's speed, and your taste.
Inspired by Junior_Djjr's Project2DFX' tutorial.

Be warned: you'll have to wait for a lot of loading screens while testing so be patient. Use Auslogics Disk Defrag (not sponsored btw) and go listen to this underrated vapor masterpiece while the game loads.

LOD
These control LODs and were the settings that got popular on LSG's newest video about GTA V. Use increments of .100000 for better results (e.g. you shouldn't use 0.150000 or 0.350000 but you can use 0.400000 or 0.500000). Use negative values (e.g. -0.300000).

1. "LodScale"
   Controls the world draw distance ("Distance Scaling" on in-game settings). It does not give that much of a boost and you might want to leave it like it is.

2. "PedLodBias"
   Controls the draw distance for pedestrians. It's an extremely subjective tweak, but what I like to do (and what I recommend) is this: when messing around with it, switch to Franklin, stay on foot, press V until you're in the "farthest away" camera mode and then look at Franklin's hair. If you can see his hair it means the current value is acceptable.

3. "VehicleLodBias"
   Controls the draw distance of vehicles. Again, very subjective tweak, but what I like to do is going to the movie theater icon on Vinewood (which is also the place in which the car on the last scene of the benchmark is) and looking to the west. If I can see high-detail cars 20m away from me, it's good.

4. "MaxLodScale"
   This line controls general draw distance and is a mix of the three values above. It also controls the draw distance for high-detail world models and it is VERY useful. I recommend using "-0.600000" and you'll have a much smoother FPS with almost 0 graphical sacrifices.
   Be careful as this value also affects the other values in the file, you should start customizing with this value and customize other values afterwards.
   Use the last scene of the benchmark or the technique to see if there aren't any LOD models on the road.

Conclusive example:
<LodScale value="-0.200000" />
<PedLodBias value="-0.400000" />
<VehicleLodBias value="-0.700000" />
<MaxLodScale value="-0.600000" />

It's easy to notice if you look for it but it's hard to notice if you don't really care about it.

SHADOWS
These settings are very useful if you have a low-end PC but the game is just straight unplayable without shadows for you.

0. "ShadowDistance" - only works if the shadow quality is set to very high, "2" should probably be enough (and might fix some bugs, as user InfiniteQuestion explained).

1. "Shadow_SplitZStart" and "Shadow_SplitZEnd"
   I did not understand these very well but the thing I've noticed is:
   if you change the numbers' position the FPS increases and the shadows have a lower draw distance (e.g if you change 0.930000 to 0.093000, then 0.009300 and so on), if anyone has a better explanation I'll be glad to know about it. If you change "SplitZStart" you should also always change "SplitZEnd" in the same way.

2. "Shadow_aircraftExpWeight"
   Same as the above but apparentely related to planes and aircrafts. Again, I did not understand these settings very well but they were quite useful.

Conclusive example:
<Shadow_SplitZStart value="0.000093" />
<Shadow_SplitZEnd value="0.000089" />
<Shadow_aircraftExpWeight value="0.000099" />

You can still notice the point where the shadow becomes low-quality but again, if you ignore it it won't be visible.

MISCELLANEOUS

1. "SamplingMode"
   This value modifies resolution without affecting the UI. It's the option "Frame Scaling" on in-game settings, you better modify it in-game. It is great if you still wanna play on fullscreen mode but without black borders. It works fractionally, so if you're playing at 720p and you set it to "2/3" the game resolution will be 480p, but the UI will retain the 720p quality.

2. "DX_Version"
   This line controls the DirectX version your game currently uses (duh).
   0 for DX10, 1 for DX10.1 and 2 for DX11.
   If you're using integrated graphics and 4GB of RAM, you should use DX10. Same thing for people with off-board GPUs but with only 512MB VRAM (e.g. the NVIDIA 8800GT). For everyone else, use DX11.

3. "ShadowQuality"
   Used for shadow quality settings. As explained on LSG, use 0 to disable them. Very useful if you have something like an Intel G43 Express on your PC (I had one of those and oh boi, it is the sinonymous for shett).

Conclusive example:
<SamplingMode value="3" /> (800x600 resolution)
<DX_Version value="0" />
<ShadowQuality value="0" />

As you can see, the absence of shadows makes the FPS increase significantly.

NOT TESTED

1. "CityDensity"
2. "PedVarietyMultiplier" and "VehicleVarietyMultiplier"
3. "<system>" section
4. "Convergence" and "Separation"

OTHERS

1. You can also check about commandline.txt, Rockstar Games itself will help you.
2. Older versions of the game have better performance.
3. Reflection_MipBlur makes the water practically invisible and has no effect on FPS whatsoever.

Edit (3 July 2021): Pardon me if this was not helpful to most people. I'm a bit ashamed of how bad my specs were back when I first made this tutorial but I still respect those that are in the same level of, if not worse than, me.
On this edit, I have removed a part talking about countries on this. It was kinda c-r-i-n-g-e, ya know?

(english is not my native language and this place seems the closest for mod suggestions so yeah, sry if I'm mistaken)

You see, there's this GTA SA mod called Trees Remove that brings a H U G E boost in FPS along with it. Like possibly many on this site, I'm a low spec gamer living on a developing country using an Intel HD 4000 and I want to play GTA V with shadows, so is there a mod that removes all the trees and bushes from the map so that the FPS goes to at least 24 while having shadows?
The closest thing I found was Props Removal from this very same site but it leaves the LODs instead of leaving nothing. Not such of a problem but I feel like my GPU can handle it with this kind of mod, and also I'm kind of new to GTA V modding but even though, if there isn't any mod like this I'm ready to learn map editing and I'm learning SA's CLEO already so yeah, anyway you get my point, you get it.

(sorry if you misunterstood anything, english is not my native language though I think I'm pretty fluent at it)
Because making a Grand Theft Auto game is hard. You see, there's this thing called 171 which is a GTA-like game being developed, but the difference is that its map is on a brazilian city. Betagames Group (the independent company behind this game) has gattered more than 10k dollars through Catarse, used lots of models from other sites (which I hope they credit them to) and it has been in active development since 2015 but its pre-alpha has released just recently.
I can't say the same for linear games though since there are some kinda nice games released, especially those based on Source engine (e.g. Portal TWTM and Hunt Down th- no, this one doesn't deserve to be called a release).