Car mod optimization is a major factor too. I enjoyed large grid racing, but I'm gaming on a ancient potato PC (i7-4790k/1070ti) so I will go out of my way to optimize any modded cars I download. (and why I'm heavily against encrypted mods...) There's a huge laundry list of things you need to do to properly optimize modded cars, but for now I will show you how to analyze cars, so you can see for yourself how poor optimized cars can heavily effect your gameplay.
First you need to enable developer mode in content manager. On the top right side click "About" below "Content Manager" you'll see a version number. Spam-click the version number until a prompted ask to turn on developer mode. (Correct me if i'm wrong, but I think you need a paid version of Content Manager) Once development mode is enabled, go to the car list in the "Content" menu you see analyze button has appeared on the bottom. Click on it will show a list of statistic such as Numbers of triangles, LOD's, textures size, skin size, etc, and color grading system so to show how optimized the car is.
The Goal here?... You want nothing less than all green A rating for smoother gameplay. (although the "COCKPIT_HR is missing" may be ignored. I don't have much problem with it personally.) Another good way to test if this true is to fill the grid with all kunos cars vs your current modded car, since all kunos cars are optimized.
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As for optimizing modded cars? Well you may wanna put aside a hour or so because as I mentioned earlier it involves many steps, such as going into CM showroom to "Unpack LOD KN5" so you can downsized each dds images, which can be done by using a free paint editor such as Gimp. afterward you'll have to imported the car model and save it over with ksEditor.
You may to use CM's LOD generator, but first you need to download Simplygon. (it's not pretty looking, but it gets the job done plus it has many adjustable settings) and for some odd reason many modders like to create 4K skin textures (Cool for replays, but not good for gameplay!) so you may want to downsize each 4k skins to 2k at least. I may give a proper tutorial one day, but at least this will give you a general idea.