While making the game, I didn't really know how quickly to scale the difficulty up, since I was worried I might find the game easy but others might find it difficult. The complexity does go up though, but IMO it doesn't really get too hard until around 100 - 150 seconds. The trees' velocity and spawn rate go up, and it caps at about 230. I was also thinking about making the player go faster or slower depending on the speed of the trees, but I didn't end up doing that.