In the current versions of Legendary and Infernal Sky II, there are some empty mob spawners which don't spawn anything. These spawners are left over from when the map was made for Beta and used to spawn the "Steve" mob. In my experience, these spawners would cause the framerate to drop heavily (<20 FPS) when looking in their direction. If you experience this issue, you may remove them, as they will not affect gameplay otherwise. It is recommended to carry out this process using MCEdit, as this will not affect the day/night cycle.

The spawners in Legendary are at the following coordinates (in the nether): End of Sanity - floating platform with skeletons and dispensers: (-125, 60, -82) (-125, 60, -80) (-125, 60, -78) (-125, 60, -77) (-125, 60, -75) (-125, 60, -73) Fluffy Bunnies of Joy - near the entrance tunnel: (-258, 65, -148) (-257, 65, -148) (-256, 65, -148) (-258, 64, -148) (-257, 64, -148) (-256, 64, -148)

The spawners in Infernal Sky II are at the following coordinates (in the overworld): Ghost Town - yellow wool building: (-249, 57, -373) (-248, 61, -387)

You can also just download the versions of the map from the Resources tab which have the spawners already taken out.