Sodium is the only accepted mod, as many other performance-based mods can mess with mob spawning and world gen. Sodium is also arguably better than Optifine, I know it is for my computer at least. If you want to read into the mod there is info in resources.

I saw from another thread that other mods in combination with Sodium like Phosphor and Lithium (what I use) are allowed, and this was confirmed by a moderator. In the "Mods" selection of a speedrun, the only mods that appear are Vanilla, Optifine, and Sodium (they just updated it to say Phosphor and Lithium yet). This would mean that BetterFPS is not allowed, and there are several restrictions in place for Optifine for 1.16+ as well, so I would recommend installing Fabric and using Sodium+Phosphor+Lithium.

Optifine isn't just restricted for 1.16+, it's outright banned. And we're looking into allowing betterfps but eh we don't need it until we get an fps improvement mod that doesn't suck for 1.14.4 because fps isn't really an issue in the betterfps supported versions