They charge fast when the battery is fresh and healthy. When the battery health gets shitty the software automatically switches it to slowly trickle charge to preserve it, although you can go into the settings and re-enable the previous fast charging.
My SE is currently at 75% of its original battery capacity. I put it on at 8AM this morning after charging it all night, tracked one workout during the day and it is now almost 10PM and it is at 16% charge and dropping fast.