I used my MagSafe battery pack for the first time today -- an outing to museums and parks in NYC with friends. I left home with 100% on both my iPhone 12 Pro and the battery pack. During the first few hours I noticed the iPhone power indicator showed it was being charged but the battery kept depleting. Oddly, the battery pack stayed at 100%. Finally, once the iPhone dropped below 60% or so, the battery pack starting depleting too, indicating that it was finally charging the iPhone. Nonetheless, the iPhone kept depleting. At the end of our day, about 6.5 hours later, the iPhone was down to 26% and the battery pack was at 75%. If I kept going, the iPhone would have depleted before the battery pack.
Final conclusion, the battery is way too slow to only begin charging the iPhone once it drops below 60% or so. Apple either needs to speed up the battery charging or allow the pack to charge the phone at a higher charge level. Otherwise, it's not a very useful product. Much worse than the battery cases of the past. I'm returning this product. A rare big miss for Apple.