Usually the more fences you jump in one exercise, the more the horse will “build”. A more experienced rider not worried about the jumps just manages the canter and controls that building speed as they go around the course.
Novices, however, ARE still worried about the jumps and its too much multi tasking to manage the canter strides down the lines, go out in the corners, find the ins and get to the outs etc. plus they get nervous when that speed starts to build and horse knows it.
If horse does not do it with a slightly more experienced rider? It’s not the horse, or, rather, the horse does know they can get away with it but nothing evil going on there.
When I went thru this, and its common, trainer put horse in a fat rubber snaffle and worked to get me more comfortable going faster instead of tensing and to package the building pace into a bigger step instead of more speed.
I got more concerned with managing pace then the jumps.
DD may not be ready for that yet so suggest a stronger rider spin her around before DD gets on and get her a little tired. And exercises over singles and halting at random mid course would be appropriate avoiding a full course where you know she’s going to light the jets. DD will get it as she jumps more and learns to manage that step.
Old Hunter saying- kick up the first line, encourage in the second line, third line should be perfect, cool your jets down the last line and throw out the anchor if its towards the in gate. Very true, they can count to 8 fences, know when they are done and know exactly where the out gate is. Not so dumb. Run and be done gets in horsey brain there too.
Oh, if trainer wants to put some lines together, take the first one TOWARDS the in gate where normally the are slugs and the second away from it as that should slow her down. Horse is just smarter about the course then DD is right now. It’ll work out. Not a bad problem, beats a stopper or putting DD in the dirt all the time or some slow slug with a double add down a line set for 10’.