So for us currently, the processing time doesn't change much depending on the frequency of the gift. But I will say that we don't invest a lot in improving this process, because overall we don't put much time into it, so maybe at an organization with a way higher volume of small donations, they would have automated some of what I described. That being said, I've worked at a handful of fairly large nonprofits that are still doing this manually, so it just is going to depend on the organization, and 5min / transaction seems like a safe bet. There are other costs besides just processing - e.g. a bit of time spent on end-of-year donation receipts potentially, etc., that you might counterfactually cause the nonprofit to incur. 

If you're doing an ACH transfer monthly, the costs will vary by provider, but they are a lot cheaper than credit cards. I think Plaid, which is a really common ACH service, costs around 0.80%, capped at $5, or something like that. There is more variation here across providers though, unlike credit cards, where there seem to just be industry standard rates.

I do think there are some genuine advantages to giving monthly, or on a recurring basis, to some organizations. A lot of EA organization keep pretty large amounts of reserves in the bank (at least, compared to similarly sized other nonprofits I've seen, it seems like). But if you're giving to a small organization that doesn't have large reserves, the cashflow certainty that comes with monthly donations might offset to a large degree the extra processing costs, etc.

If I have to guess the best way to give to reduce costs, stress, etc. on an organization, I'd guess it would be a lump sum given in maybe March-May, when the fundraising season is in a lull, but I really think that's probably only very marginally better than giving in whatever way works best for you, monthly, etc., so if monthly helps someone give with less stress, it's probably still worth doing. And if it is a small organization, monthly can really help.