Haven’t done the dyfi but my 10utb experience was like that, first lap wasn’t just riding in a line, it was quite often standing completely still, in a line, 500 people waiting for a roadie-on-knobblies half a mile ahead to carry their £10000 carbon bike over a fir cone. Only livened up by a brief fistfight which was something to do with standing in lines. Never doing that again, it was dismal.
It really doesn’t have to be like that, GT7 frinstance is another big mass start event (maybe not as big?) I’ve done 3 first laps and never really felt badly held up on any of them- sure, for short spells but nothing like that. They give you a very long fireroad climb at the start to split the pack, the entire singletrack climb is bypassed. On paper it’s a boring lap but in practice it’s far better.
(yes yes, if you’re super fit you can just get to the front, but most people are going to be in the mob)