The StepSmash app does send data in chunks, but it’s not usually a slow process. Samsung Health is one of our most popular integrations, and across the folks using it, we haven’t seen any major issues. When the next version of StepSmash is available next week, the additional logs we’ve added to the application will help us determine what is going on with your account specifically, so we can get to the bottom of what you’re seeing.
As far as Samsung Health vs. Health Connect, the only major difference is how Active Minutes are determined. When syncing from Samsung Health, Active Minutes are mapped directly to the Active Time metric in Samsung Health. When syncing through Health Connect, Active Minutes are calculated through a combination of heart rate, age, and resting heart rate. You can find a full description of how that works here: How Active Minutes Work in StepSmash