A mob that is loaded, is loaded. No difference where. If all of those mobs are actually in your sight, you might get lower frames due to graphics rendering.
Endermen are part of the overall hostile mobcap, there is no separate mobcap for them.
The mob cap for endermen is (70 - the number of other hostile mobs currently in existence).
Note that in 1.16 endermen are one of the mobs that use the new spawning zone mechanics, which prevents other mobs of the same type from spawning in a small area around them. These areas meld together to form one bigger area encompassing all members.
Having many mobs within a small area, close enough so they collide, especially within a single block, can cause server lag due to the collision calculations (Spigot even has a setting, max-entity-collisions, which limits how many entities an entity can collide with to reduce lag) but aside from rendering there won't be any direct effect on client-side performance unless you have a very old CPU with only 1-2 cores, but they aren't suitable for running the game in any case. This is also likely why Mojang added entity cramming, which effectively limits collisions and lag by killing them. It should also noted that the server only sends entities within 80 blocks of a player to the client, so with normal mob spawning a significant percentage of mobs will be further away (because of this, F3, which only shows client-side entities, is not a reliable way to see if you are below the mob cap).
Is there a difference for performance if there are 100 mobs in 1 chunk or if they are spread across 10 chunks? If yes, what is faster.
And a question to the mobcap: what is the mobcap for enderman?
A mob that is loaded, is loaded. No difference where. If all of those mobs are actually in your sight, you might get lower frames due to graphics rendering.
Endermen are part of the overall hostile mobcap, there is no separate mobcap for them.
The mob cap for endermen is (70 - the number of other hostile mobs currently in existence).
Note that in 1.16 endermen are one of the mobs that use the new spawning zone mechanics, which prevents other mobs of the same type from spawning in a small area around them. These areas meld together to form one bigger area encompassing all members.
Having many mobs within a small area, close enough so they collide, especially within a single block, can cause server lag due to the collision calculations (Spigot even has a setting, max-entity-collisions, which limits how many entities an entity can collide with to reduce lag) but aside from rendering there won't be any direct effect on client-side performance unless you have a very old CPU with only 1-2 cores, but they aren't suitable for running the game in any case. This is also likely why Mojang added entity cramming, which effectively limits collisions and lag by killing them. It should also noted that the server only sends entities within 80 blocks of a player to the client, so with normal mob spawning a significant percentage of mobs will be further away (because of this, F3, which only shows client-side entities, is not a reliable way to see if you are below the mob cap).
TheMasterCaver's First World - possibly the most caved-out world in Minecraft history - includes world download.
TheMasterCaver's World - my own version of Minecraft largely based on my views of how the game should have evolved since 1.6.4.
Why do I still play in 1.6.4?
will endermanfarms work in 1.16? From what I heard the endermanspawning only changes in the Nether