My guess is your average speed is around 7-10 miles an hour,not bad for city streets.I ride a Brompton and I use a speedometer that measure everything,the average speed for 6 miles is around 10 miles per hour,even though I can do up to 15-20 on down grades.Most of the time is taken up by stop and go,slowing down to avoid other,etc.Don't forget you just started riding to work, in time ,your average speed will improve with practise.