Setting biomes using World#setBiome, delegating to CraftWorld#setBiome will create a new ResourceKey every time a biome is set, using String#intern on the biome ID obtained from the bukkit biome, to convert it to an internal BiomeBase. This is an extremely resource-costly method and setting relatively small regions of biome (in this example a 91x91x384 region) can have a massive performance impact on the server.

PlotSquared uses a chunk-by-chunk approach that attempts to place a chunk at a time, without overloading the server, however the repeated use of String#intern overloads the server very quickly leading to very high tick times when biomes are being placed.

Timings: https://timings.spigotmc.org/?url=alalexubuc
Spark profile: https://spark.lucko.me/60E2fYza9B