The parfor implementation already attempts to take account of this sort of situation - behind the scenes, the loop is split up into multiple "intervals" or sub-divisions of the overall loop range. The interval sizes are chosen using a heuristic that attempts to get decent performance in most common situations. Each worker gets roughly 5 intervals (providing there are enough loop iterates). These intervals are not equally sized - we start with short intervals to get the workers busy as soon as possible, then some larger intervals to try keep the workers busy with decent amounts of work, and finishing with shorter intervals to try and overcome effects like you're seeing in cases where the iterations take differing amounts of time.
In particular, we don't send all the intervals out to the workers up-front - if one worker gets an iterate that takes a huge amount of time, it will not receive any further intervals from the client.
If you want more control, you should investigate parfeval as this lets you divide up the work as you see fit, at the expense of not being quite so convenient as parfor.
