When I do the web site test. the pacing time(the time interval between the start of a transaction and the start of the next transaction) is very difficult to determine.

At first, I do a smoke test with one VU, 10 transaction to find out the avg respond time for one transaction.

For example, the transaction can finish within 70sec. then I assume that one transaction will run per 2min. so the pacing time I set is 2min. The avg respond time is shorter than 120sec when the VU is no more than 30. But if I use 60 VU to hit the web site. the respond time will increase and it will be longer than the pacing time.
At this time, each transaction will run one by one without any pause.

Is this phenomenon not realistic if I do not change the pacing time so that there are some pause time between two transactions?