I think the gap between your fresh run time and triathlon run time is down to overall swim, bike and run fitness rather than effort. Or at least that is the biggest factor. The fitter you are in all 3 the smaller that gap will be.
As @explorerJC asks, if you felt you went as hard as you could have, you probably did. A reduced HR in the HIM shows that centrally, your body found it less stressful to produce that speed.
ETA: [quote=“joex, post:16, topic:1951”]
If 12bpm lower indicates a normal level or high level of fatigue for my half run, then my problem lies somwhere else.
If 12bpm lower indicates a lack of effort or a low level of fatigue, then the problem is in my run.
Where the gap in fitness lies is difficult to assess with only run info. But the reduced HR is directly because of fatigue, assuming you’re trying which I’m sure you were. My first IM marathon, my avg. HR was 135, my 2nd it was 150 I was fitter and importantly the bike was cancelled. So I had a bit of fatigue from the swim but obviously not the cycling so my HR was nearer a fresh marathon avg HR.