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.
Demonstration video

Proposed solution:
Modify Slime spawning rule JSON to include a spawning height limit of 39 in all biomes but swamps, where the limit should remain 70.

Edit:
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.