Just lower the speed for the egg whites so that they are whisking very slowly while you wait for the syrup. You can also stop the mixer for a couple of minutes, but make sure you re-whisk the eggs on low speed just before you add the syrup. Alternatively, if the sugar syrup is ready before the egg whites are, just turn off the heat of your stove and let the sugar syrup sit there for the minute or two that it takes to whisk the egg whites to soft peaks.