I have never noticed this myself.
A lot depends on expert advisor, because some of them do a lot of calculations therefore testing takes more time. If you have your own advisor that was edited for both mt4 and mt5, and has same functionality in both terminals - then you can compare these versions to see speed difference.
Also, MT4 terminal has special tab that displays all trades during testing, it probably takes some time for MT4 terminal to process and display this trade data.Also, MT5 can have something like a better signal result caching, which can speedup everything too. But this is only guess.