A lot. 70 seconds worth in my case. Shoe design is of course part of it. But the weight of the shoe is the most noticeable thing when I’m wearing the brand new lightweight New Balance 730. The mile time trials were about a week apart. I’m still very slow in the mile. And all other distances for that matter.
So if you’re getting back into running, or just plain wondering what lighter shoes might do for your running performances, buy some lighter shoes and reap the time benefits!

1.6lbs for the old New Balance. They're still in good shape too so having worn out shoes is not the case for the newer shoes being so much better.

These are the new lightweight New Balance 730s. They feel like racing shoes to me. But I'm used to shoes that are more on the clunky side of things since I haven't been a runner in a long time. But that changed a few weeks ago and I have been running after work a few days a week now. I was running a bit before this - maybe 10 times this year before starting to run with the group - but now I'm taking it more seriously and I have some motivation and camaraderie while running. Which is so much better than always running alone.