The following error regularly appears in the logs/console. Entities seem to be severely impacting server performance lately, and I wonder if this error has anything to do with it.

[06:32:01] [Server thread/ERROR]: "Silently" catching entity tracking error.
net.minecraft.server.v1_9_R1.ReportedException: Adding entity to track
at net.minecraft.server.v1_9_R1.EntityTracker.addEntity(EntityTracker.java:142) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.EntityTracker.track(EntityTracker.java:81) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.WorldManager.a(WorldManager.java:18) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.World.b(World.java:1015) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.WorldServer.b(WorldServer.java:1120) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.World.addEntity(World.java:1008) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.ChunkRegionLoader.a(ChunkRegionLoader.java:467) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.ChunkRegionLoader.a(ChunkRegionLoader.java:463) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.ChunkRegionLoader.a(ChunkRegionLoader.java:474) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.MobSpawnerAbstract.c(MobSpawnerAbstract.java:106) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.TileEntityMobSpawner.c(SourceFile:61) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.World.tickEntities(World.java:1485) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.WorldServer.tickEntities(WorldServer.java:628) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.MinecraftServer.D(MinecraftServer.java:792) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.DedicatedServer.D(DedicatedServer.java:400) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.MinecraftServer.C(MinecraftServer.java:660) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at net.minecraft.server.v1_9_R1.MinecraftServer.run(MinecraftServer.java:559) [spigot.jar:git-Spigot-f04e043-52c7fc0]
at java.lang.Thread.run(Thread.java:745) [?:1.8.0_74]
Caused by: java.lang.IllegalStateException: Entity is already tracked!
at net.minecraft.server.v1_9_R1.EntityTracker.addEntity(EntityTracker.java:109) ~[spigot.jar:git-Spigot-f04e043-52c7fc0]
... 17 more