Why is mySQL running unpredictably?

Question

MySQL

I will give an example: and the table has 800 thousand lines.
SELECT * FROM `users` WHERE age = 27
SELECT * FROM `users` WHERE age >= 27

Since there is no key in the age field, it will iterate over ALL 800 thousand rows in order. EXPLAIN proves it.
But what's interesting.
First select: (11269 total, Query took 0.0024 sec.)
Second select: (597449 total, Query took 0.0003 sec.)

SUMMARY: First select was 8 times slower!
Even if you make more complex queries, then the same topic. Recently I made a complicated request.
In the first case (=) the query took 0.02 seconds, and in the second case (>=) the query took 0.0005 seconds.

Why is this happening? what mystic?)