High is not a faster low
Cooking on the slow cooker's high setting instead of low badly hurt tenderness (2/10 in both reports that tried it) -- low-and-slow is not interchangeable with high-and-fast for this dish.
Every recipe on the internet prints "8 hours on low or 4 hours on high" as though those were two routes to the same dinner. In our reading they are not. Six cooks on low rated the beef 8 or 9 out of 10; the two who tried high rated it 2. Connective tissue needs time at a moderate temperature to turn into gelatin. Push the temperature up and the muscle fibres contract and wring out their moisture faster than the tissue softens, so you arrive at dinner with beef that is tough and dry at once — the two failures people usually think of as opposites.
Using a slow cooker/crock pot on its low setting (including transferring to one after browning) reliably produced very good to excellent tenderness (8-9/10) across six independent reports.
What to do: low, and start earlier. If it is already on high, turn it down now rather than shortening the cook.