1.18.12 Hotfix, 1.18.31
Despite the fact Slimes are only supposed to spawn in slime chunks at Y<40 and swamps at Y=<70, spawns are attempted by the game in every biome, every tick, with a weight of 100.
As a consequence, spawning rates for other mobs, especially mobs with a low spawning weight, are greatly impacted in a negative way. This is especially clear with certain passive mobs.
As an example of this behavior, here is a video using turtles in a beach biome.
The left side of the video shows plain vanilla Minecraft.
The right side of the video uses a behavior pack which limits slime spawns to Y<40 in all biomes but swamps, where the upper limit is set to Y=70.
The results should speak for themselves.
In vanilla Minecraft, during the 5 minutes of the test, only 3 turtles were successfully spawned.
With the adjusted behavior pack, 33 successful spawns were recorded in the same amount of time.
Modify Slime spawning rule JSON to include a spawning height limit of 39 in all biomes but swamps, where the limit should remain 70.
After testing the pack uploaded by ArchThunder and confirming it produces similar results as the one I initially attached, I decided to remove mine in favor of theirs. ArchThunder's pack is very close to the concise version proposed by GoldenHelmet.