Despite any confusion that you might see in the media, any delay in reporting results in Maine or San Francisco is not caused by the ranked choice voting system. These are normal functions of how elections work, depending on the rules set up by each state.
For example, Maine elections are never decided on Election Night. Maine is a large, mostly rural state, and elections are not run by the state itself, but by the 500 or so municipalities, which often take a week or more to get official results to Augusta. For RCV, that means that while most first-choices have already been counted, the round-by-round count will not be able to take place until early next week.
That would likely not be the case in most other states, which generally run elections at the county, rather than municipality, level, and transmit encrypted ballot data electronically, something that Maine state law prohibits. Once the votes have been reported, however, the actual round-by-round tally takes only a few minutes.
In California, election results routinely take a long time. San Francisco is actually faster than most counties. Interestingly, San Francisco accepts ballots with an Election Day postmark, even if they arrive well after Election Day.
This could result in extending the time it takes to get the ballots counted. For its June 5 election, there was a tremendous surge in vote-by-mail ballots dropped off on Election Day, all of which need to be inspected and verified. All 58 counties in California, regardless of the election system, have still been counting ballots after the election. As with Maine, the round-by-round count is quick once ballots are processed.
For both San Francisco and Maine, the election administrators are only limited by the rules in place, not by ranked choice voting. The election results would take a similar amount of time under any voting method.
You know what takes even longer? Holding runoff elections.