inaccurate waiting loop time

[zmn28hgbn59kcmlpio8unfh7fdre523esd28q9a]

when setting server speed to x5, waiting loop takes more than the required time.
example:
say two building orders each takes 1 minute, in theory waiting loop should be:
00:01:00
00:02:00 (waiting loop)

however it is not the case, waiting loop can be as much as half a minute longer e.g. 00:02:30 instead of just 2 minutes.
i have no clue where this happen tho, not that i can fix it but at least i could try.