You can see these kinds of differences with just a few seconds difference, ideally I aim to stay under 2s, even on the slowest connection type. 2s is already very long for a user to wait and many will not.

Non profits are tricky. You could see volunteer sign ups and donations as conversions. I manage a non profit site as well and unfortunately I don't have a good solution that is both fast and approachable for our staff to use, so we had to make that compromise as well.