The validation process does for every record a count query to check if the record exists. This is regarding the use of the 'on' => ['create'|'update'] validation functionality. However this isn't always needed; only when the 'on' key is actually used. Which you won't be using all the time.
This pull request optimizes this validation process where the 'on' key isn't used.
Not sure if this is taken care of in 2.0, but I can imagine you'd like to port it.
Added test for the 'on' key in validation rules.
Optimized the validation processes when 'on' keys aren't used.
P.S. The error/fail the model validation test cases gives was already there. I checked with @Ceeram.