If you are travelling midweek-ie Monday to Friday-leaving Stanstead
at three thirty you will hit the M25 at about its busiest period,all the way round to the M4.And the M4 will be heavy traffic heading out of London too.Unless there are serious road works or an accident,the traffic does move,though you wont be cruising at 60 mph.
You could head Cross Country,but the difference is probably negligible
and if you miss a turning you could waste hours.I would stick to the
motorway route,or even delay departure from Stanstead till six or seven o clock.