I usually prefer to ride sweep when we have our training rides as well. And yes I feel like it's more difficult than just going at the pace that's comfortable for me. Extra stops are harder than just keeping on going. But I find it much more rewarding and enjoyable because the folks who are back there really need and appreciate the assistance. What I will often do, especially if we are going to be on the same road for a long time is hammer ahead to join the faster group, let them know how many I have back and then go back to join my ducklings, or sometimes I'll add some time on after everyone is back, just to give my legs a good spin out. That has helped me avoid being sore or extra tired later. The slower pace also allows me to be able to get a good look at the riders and their positioning and help them with adjustments to make them more comfortable and efficient cycling machines.