After playing around with the collision of multiple entities, I've come across a strange behavior in the slime collision.

To run these tests, I've set up these command blocks on a repeating command block clock:

/tp @e[tag=kill] ~ ~-512 ~
/execute @a ~ ~ ~ detect ~ ~-.1 ~ minecraft:packed_ice -1 summon Slime ~ ~ ~ {Tags:["kill"],Invulnerable:1b,Size:5}

This will constantly summon and kill a size 5 slime at the player's position if they're standing on packed ice.

One would normally expect to be pushed at similar speeds in all directions, if they stand on packed ice. This doesn't seem to be the case.

I've created a path (see path.png) of packed ice, blocked off by some glass, and surrounded by a chunk grid. This allows me to test the commands out and track the results.

Once you have the command chain running, you can hop on the path and try it out. You will begin to be pushed by the slimes. From my results, it takes about 3 seconds to cover a single chunk going towards negative X at terminal velocity, without pressing anything. On the contrary, it takes < 1 second to cover a single chunk towards positive X. The same behavior is repeated in the Z axis.

I have repeated the tests on other mobs, and only this combination has given me inconsistent results.

This is a short gfycat demonstrating the bug in question:
https://gfycat.com/UnhealthyBronzeKakarikis
(Note that the only time I touch my keyboard is to start and to switch directions)

Note: I've also repeated these tests in other worlds as well, without any command blocks running other than the ones needed.

This represents a lack of consistency within the game, and makes contraptions that work with slimes much less controllable than they should be, let alone all other bugs with them.

EDIT: After working with slimes once again, I've concluded that it not only happens with ice. Regardless, packed ice makes it easier to show.