I don't have a super powerful computer, so playing with a render distance higher than 12 is awful with frequent stuttering and fps drops, even with Optifine. However, there's a new mod called Sodium that offers dramatic performance improvements over Vanilla and Optifine called Sodium. Here's a quick video comparing the performance of all three:

Sodium optimizes the rendering engine, but unlike Optifine it doesn't change gameplay because it doesn't have any unnecessary features (eg. Zoom, changing particles/fog, extraordinary render distance, changing the chunk loading ie. lazy chunk loading, dynamic lighting etc.) All of its changes are limited to performance only. Sodium can be paired with another one of jellysquid_'s mods called Phosphor, which optimizes the lighting engine. This helps when generating chunks but doesn't make much of a difference otherwise.

For example, I went from 11-20 fps during initial world load on vanilla to 50+ with sodium. Here is another direct comparison between Vanilla, Optifine and Sodium on my machine: - 32 chunk render distance Vanilla on default settings. Awful. Stuttering and 1/2-1 second freezes. ~20 fps - Optifine on default settings (w/o vsync). A lot better, but still has stuttering. Note that some things look different to vanilla (ocean on the far left) ~40 fps - Sodium + Phosphor on default settings (w/o vsync). Some minor stuttering but otherwise good. ~120 fps

I suggest allowing the two mods for 1.16+ speedruns because of how poorly optimized the game has gotten recently. They essentially do the same thing as Optifine (but dramatically better), and without the potential for unfair advantages. It would greatly improve the buggy and laggy 1.16.1 experience for a lot of speedrunners, especially because I doubt many of us will update to 1.16.2 for the performance improvements because of the bartering changes. Perhaps most importantly, it evens the playing field between those with good computers who can comfortably run at 20+ render distance and the rest of us.