To me, this part of the mod is more of a proof of concept, as I won't claim to understand how grass in the game works in general, but I will simply state my observations in the json code and in-game.

In an unmodded game, grass is fairly static except in a couple of instances.  One instance is if the player cuts grass to an "eaten" state.  After 200 in-game hours, it will grow back to medium height.  Simple.

I altered this same mechanic to allow it to grow in stages instead.  Eaten will now grow into Very Short Grass, and that will grow into Short Grass, continuing until it is Very Tall Grass.  I have set the interval to be about 50 in-game hours between each stage.   So it will take at least 300 in-game hours (12.5 in-game days) for Eaten Grass to grow into Very Tall Grass.

I tested several worlds to see how this would affect it, both in instances and performance-wise.  I will claim the fact that I'm not very adapt at running performance diagnostics, and will simply state the following as my observation. 

In short, World generated grass does not seem affected by this.  Only grass that were cut to an eaten state and grass that newly spawned on "disturbed" soil seemed to inherit the code I established.

In normal testing, I generated new survival worlds to first observe if grass grew in general.  After I concluded that it didn't, I took notice to my system resources using task manager, switching between my large mod list to a more vanilla setup.  In short, I couldn't discern any slowdown or studderings traced to this mod.  (I did have the game max out my 16GB of RAM at one point, but the logs indicate this is likely caused by Butchering and Detailed Animals butting heads and creating constant errors related to animals).

In a more extreme test, I created a creative flat world, placed as much eaten grass around me as I could (at least 300 blocks in each direction, the paintbrushtool is good for this), and fast forward time.  There was no significant RAM usage when grass updated in growth, just a temporary 10% bump in CPU, but this is likely normal with advancing time rapidly.

Would this cause very tall grass to be everywhere?  That would be annoying.

No, it seems to only affect grass that are disturbed directly (cut to an eaten state) or in-directly (grass that newly spawns on recently placed or fallen dirt) by the player(s).  So if anything, your yard is the one at risk if you don't keep it trimmed.  :)

Would this affect performance in any way?

It seems unlikely, or at least only marginally if at all.  My tests, in both an largely unmodded and heavily modded environment, did not show any significant increase in hardware resources, including RAM due to this mod.  This is likely due to the previous.  However, I possess an above average computer build (Ry5 5600X, RTX4060, 16GB RAM at time of testing), so its possible weaker systems may notice something.

When and how will grass grow?

Grass will progress in their growth stage about every 50 in-game hours (just over 2 days).

Just like in vanilla, grass won't grow in tempatures under 8 degrees, and will entirely reset their timer under -1 degrees.