It's an inherent problem with optical HR. Go to the Garmin forums and read the many, many posts there about this on their devices. While it appears to work ok for some, it doesn't for many.

Not sure you'll find much joy anywhere on any device with optical HR.

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Trust me I’m a doctor!
Well, I have a PhD :-)

I too have gone to the Fenix 5s a few weeks ago. I don't notice any lag with the OHM. I've compared my 920 with a chest strap HRM and the F5 with the OHRM and they correlate somewhat well. Definitely don't see any lag. In fact, there are times where I feel the OHRM responds more quickly or possibly Garmin put in a bit of a filter when using a chest strap HRM.

That said...the OHRM sucks!!! I now wear it on the inside of my wrist to make better optical contact. It works, but any time I really jar the watch (like bumpy roads), I will sometimes lose just enough contact that I get a false reading. The FW then seems to have this filtering so the HR stair steps back to 'normal'. Mostly on average I get a few bpm more using the OHRM which I think is due to the occasional spikes up.

I do like the OHRM to monitor my 24/7 HR however, so not a total loss. When doing a hard workout, I'd definitely switch to my chest strap HRM.

Sounds about right -- I'll stick to the strap for workouts. Thanks guys!

Me to. That's why I bought the Fenix 3 wihout HR. Seems that that also supports battery life.

My F5 optical HRM simply doesn't work on workouts. The HR it shows is nowhere near where it should be so I just use the Garmin HRM Tri.

No idea why - I've tried to tighten it, and loosen it, and move position. Just doesn't work properly.