Probably affects all 1.14 releases, but debug reports from 1.14.4 pre-release 1 were analyzed.

Mob AI seems to be the most important cause of performance issues (especially cod AI when near water). 

I've created three debug report under three circumstances:

1. Swimming in an ocean - 8.98 ticks per second, ~25 FPS
2. Walking on land - 16.79 ticks per second ~50 FPS
3. Walking on land with doMobSpawning and doMobLoot both set to false, with a custom datapack that runs 

   tp @e[type=!player] 0 0 0

   then

   kill @e[type=!player]

   every tick. - 18.83 ticks per second ~55 FPS, often above 70

In case #1, cods seem to account for 52.53% of the tick time.
In case #2, no type of mob seem to take more than 13.30% of the tick time.
In case #3, chunkSource takes the longest but the game is now playable.

Sorry if I interpreted any of the debug report info incorrectly. The datapack (kp-disable-entities.zip) and the three debug reports are attached.
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Extra info: fancy graphics, smooth lighting maximum, mipmap levels: off, render distance: 12, clouds: fancy, particles: all, entity shadows: on, VSync: off