Spawn rates are dispersed among all players that are currently logged in to the server. If you keep an eye on your farm, you will notice a vast difference between when there is 5 players online, and 20.
For each player that is currently online, there is a valid spawning area around each player.
As an example, let's say the mob cap is 100. If there is 5 players online, then that means ~20 mobs will be spawning near each player, including you at each interval. If there are 20 players online, that will drop to ~5.
Depending on where each player is can affect whether or not mobs can spawn, for example if a player is in the nether, or in a well lit area, this reduces the amount of mobs that can spawn near that player, which can increase your farm's spawn rates. If a player is in a cave, or out in the wilderness during nighttime, mobs will be able to spawn around them, reducing your farm's spawn rates.