Opening the original SkyBlock 2.1 map in 1.13 results in lagspikes on the internal server.
If someone recreates the map with the void biome the lagspikes are gone as in this biome no mobs will spawn. But no mobs == no fun.

Crosschecking the map with 1.12.2 results in a smooth gameplay and no dreaded "Can't keep up!" messages in the log.

As there is no TickProfiler or similar tool for 1.13 yet i can not help any further.