How good is the built-in HRM for intense activities and cycling?

I'm quite OCD when it comes to HR and after years of frustration with the Elevate v1 HR sensor (FR235), v2 (FR935 or your F5+) I found with the arrival of the Elevate 4 sensor (on my Venu 2 and now FR965) that it was very reliable for almost any kind of run provided you wear it tight, ideally with a nylon strap. It still lags a bit compared to an HR strap so that could be an issue on very short intervals (which I don't do). YMMV of course.

I would recommend looking at the reviews by DC Rainmaker and/or DesFit on the Forerunner 965. They have conducted extensive testing on all sorts of devices.

Thanks, that's encouraging. I'm curious about why you recommend a nylon strap - please can you expand on that? 

I do read the DCR reviews. Thing is, with this aspect, there are so many variables that can potentially affect the outcome (skin colour, hairiness...) that it'd be useful to know the experience of as many people as possible.

Ive been using garmin with heart rate strap for many years and different devices until i got the 965. My heart rate strap broke recently so I just decided I don't care so much anymore about accuracy and gave the OHR a try. My training also changed a lot from speed and intervals to mostly long and slow efforts.

Basically it's  mostly fine until it has a bad day and then all of a sudden it's a big mess until you restart the watch. Or the other day for example it was cold out -1c and i went for a 45m walk in the Forrest,  my avg heart rate was reported as 69 which... is total bs, for sure . Other times it feels spot on, i guess. If I would still be training the same way I did before with intervals and stuff I would by a strap for sure. It's just much more reliable a the time, but a bother to wear it.

Hope this helps.

You cannot say “how good is the optical sensor in general”. It varies between people a lot.

I can share my experiences.

For interval it is rubbish. Totally unusable. And I do interval training a ,ot (twice a week at my atletics club). So, this is an important one for me.

For long runs, it depends. In summertime it’s good enough to get an indication of effort.

In wintertime, things are different. I get almost always cadence lock.

For cycling, it is fine. Do say, I mainly use cycling for recovery training and do ‘easy’ rides for up to 100k. I do climb a mountain occasionally, but then I don’t care about heartrate. 

For running I use a stryd and a cheststrap, and for cycling I use a power meter. That gives me a better indication.

But again, this is my experience. It doesn’t say yours will be the same. Yours might be better or might be worse. I would say, don’t depend on it, buy a cheststarp.

What you describe was true for me with the Elevate v1/FR235 and v2/FR935 but that has not been my experience at all over the past 3 years with the Elevate 4 sensor, first with the FR955 then with the Venu 2 and now with the FR965. In my experience over 750+ activities it's been more than 95% accurate with the rare "lapses" explainable by heavy sweating (not sure an HRM strap would have been any better) and extreme cold.

it just happened this week, FR265 (elevate 4), during a relatively intense brisk walking, blue is the pace

the watch needed 5 minutes to realize, "ahh, something is going on!"
and at 2 minutes mark, my HR started even dropping, despite walking fast.