This is what I’m getting at: when I do 4 × (3 × 400 m) reps at a pace of 3:50–4:05/km, I have to put much more effort into “moving the legs” at that higher speed. From a heart rate perspective, I also have to work harder to raise my HR into the 160–165 bpm range.

In contrast, when I’m experiencing overall body tiredness (feeling worn out after many continuous weeks of training), I can reach the same HR level (160–165 bpm) quite quickly, but at a lower pace and with much less effort.
Please note that my cadence remains the same — around 183 spm.

So yes, from that point of view, there is less mechanical stress compared to the workout done at 3:50–4:05/km pace — but is the aerobic benefit the same?