Game isn't running at 20 ticks per second

Normally the game runs at 20 ticks per second, but it's running slower than usual.

Steps to reproduce:

1) Set these commands:

/scoreboard objectives add Ticks dummy

/scoreboard objectives setdisplay sidebar Ticks

/scoreboard players set @p Ticks 0

2) Place a repeating command block set to Always Active and write this command:

/scoreboard players add @p Ticks 1

3) Notice that the speed of a game tick is different at higher y levels (almost feels as if it's at 10 ticks per second at y=200)