There is potentially a visual trick being played on the user where are lower frame rates (which 100+ AIs certainly causes) where the hardware cursor is still moving at full windows framerate, but the camera is turning at the rate the game processes which, at lower frame rates, is certainly slower. With the cursor turned off this difference is not nearly as noticeable as it is when you can see how quickly the cursor moves across the screen.
Camera Movement is laggy while the mouse button is held down and ShowMouseCursor is enabled if there are AI moving. Stat Scene Rendering didn't show any jumps while moving the camera under these circumstances.
Repro steps are for recreating in a new project, but I also attached an example project. In the example project, press M to toggle ShowMouseCursor. Note the difference between when the cursor is shown and when it is not shown, if the mouse button is held down and moving the camera.
Reproduced in 4.8.3, 4.10.1, and Main (CL# 2802569)
1. Create a new Third Person template project
2. In the ThirdPersonCharacter Event Graph, set a key input to Get Controller > Cast to PlayerController > Set Show Mouse Cursor (True)
3. In the Level Blueprint, create a new Custom Event
4. SpawnActor Third Person Character > AI Move To
5. Give the SpawnActor node a valid Spawn Transform on one end of the map
6. Give the AI Move To node a valid Destination on the other end of the map
7. Set a Timer by Event on Event Begin Play
8. Set the Time to .30
9. Set Looping to True
10. In the ThirdPersonCharacter BP, set Auto Possess AI to Placed in World or Spawned
12. Once there are roughly 100 AI bots on the screen, move the mouse around
13. Press the input key to show the mouse cursor
14. Move the mouse around while holding down the left or right mouse button
Extremely noticeable lag in the camera movement once a number of AI are moving around. Let about 100 AI get moving (they don't need to be immediately visible in the viewport), and the lag is obvious. Note that this is considerably worse than anything caused by the AI itself: if the cursor is not visible or the mouse button is not held down, moving the camera around does not reproduce this lag.
Head over to the existingAnswerHub thread and let us know what's up.