I’m not entirely sure what’s being asked here. Is this asking “if we do experiment 1000001 and see k Rs in the first four trials, then what credence do you assign to the 5th trial being R?”
Or is it “if we take a random experiment out of the million and see k Rs in the first four trials, then what credence do you assign to the 5th trial being R”? This isn’t the same question as the first.
Or is it something else again?
There aren’t discrete “splits” in quantum mechanics, and consequently no way to accelerate them. Branches are a lies-to-children simplification that you can employ in a few toy scenarios, but fail to be useful in pretty much everything else.