Going to a 34 from a 39 would be similar to adding one more lower cog in the back.
See this Mike Sherman's Gear Calculator:
52, 39, 34 chainrings with a 12-25 cog set.
See the
Speed over RPM Range chart -- the 34 chainring is in red, and the 39 chainring is in black.
(It'll pop up a message that you can save the current page in your favorites, but just click OK--nothing happens.)
You can try different cogs, chainrings, and cadences.
All the charts and graphs update immediately.
I like my Campagnolo 34-29 low gear. Even on 8% grades, I usually don't need to stand up. And on 4% grades, I can spin at maybe 80-90 rpm.
When I rode up the fairly steep Mt Mitchell access road from the Blue Ridge Parkway, I was doing a reasonable 60 rpm at about 5.5 mph, and keeping my heart rate at a sustainable level. If I had a 39-25, it's way down to 45 rpm for the same 5.5 mph. That's really hard on my legs.