While slowing mortar cure time can help, calling it the main fix might miss other factors. I've seen joints fail more from poor mixing than fast curing, especially in humid conditions. Getting the basics right first usually leads to more reliable results.
Yeah but so much comes down to the weather. That slower cure is a game changer on a winter job where nothing wants to set right. I rushed a patio once because the forecast said rain, and the mix was way too wet. The joints just washed out. It's never just one thing, you need the right mix and the right time to let it be.
So how do you balance the slower cure with getting the mix right from the start, especially with what @the_beth said about weather throwing things off?
But honestly, the slow cure can become a crutch. If you nail the mix from the start, the weather matters a lot less. I've seen perfect joints set fast in bad conditions because the ratios were spot on.