How much easier? Simple math. 39/34 = 1.15 or 15% easier. A typical 1-cog change is in the 8-11% range. If you need more help, then a cassette with the next larger cog size is needed.

How much slower - same answer. 15% in the same cog at the same cadence, but the easier part may allow cadence to increase.

FWIW, I'm 57 years old and can handle most Colorado mountain climbs with a 34/25. There are a few where a 34/27 would be handy. I usually shift 2-cogs smaller when I get out of the saddle. If my 34/25 isn't comfortable when seated, I'd drop to the 21T and stand.