UPDATE 6.1.19: I wasn't expecting this video to get a few thousand views, so I'm going to add more details here for those viewing this and still considering a purchase. When I did this, I had three Garmin Fenix 5x Plus models. I exchanged each of them thinking that the predecessor was broken. On all three variants, the slow and inaccurate HR measurement was present. What you should know is that the Garmin Fenix watch is useless for any type of interval work unless you pair it with a chest strap HR monitor. If you plan on doing any activity that has a rapid surge in intensity that will end in under say, one minute, the watch won't be able to register your heart rate. It's also really slow when you begin a cycling workout. The Apple watch series 4 is far superior.
I did many more tests than what I posted here. I would sometimes sprint on my bike for one minute intervals and the Garmin watch would never read my HR. It would stay at 89 when it was truly 170bpm. I've done triathlons with it and I have regularly ridden my bike 30-40 miles at a time and ran. During the steady state stuff, it will measure an accurate HR after about 4-5 minutes of warming up. What it also consistently fails to do is accurately represent heart rate as it descends. If you do a ride and suddenly stop, it will take several minutes for it to reflect your lowered HR. Usually it was 40bpm higher than my actual HR. After 3-5 minutes it would drop down to a more accurate value.
I also went on several runs where I would sprint for 30 seconds and recover and once again, the watch rarely could not take a reading fast enough to tell me what my actual BPM were. Once again, the Apple watch, miraculously was able to keep up with and provide me with accurate heart rate information, even during those short, high intensity activities. Most wrist based optical HR monitors are known to fail in the HIIT category because they are too slow to measure, but that simply is not the case with the Apple watch series 4.
Garmin should be the gold standard, especially with the prices they charge but they aren't. Their HR hardware and algorithms are unacceptable for any anaerobic activity of short duration, and they fail to accurately represent your heart rate in a timely fashion as it descends once an activity ends. Each Garmin watch I received as a replacement was slow to catch up when I increased the intensity of an activity and egregiously slow to reflect a drop in HR after an interval or sprint was over. It would take several minutes for it to accurately display my lowered heart rate once I stopped to rest. The Apple Watch series 4 was as good as the chest strap 99% of time. Start or finish. It was EXTREMELY rare for me to notice any delays once it locked onto my HR.
Wait for the Fenix 6 and hope something changes, or spend more money and use a chest strap monitor. I still do not believe a product costing 600-1200 should require any peripheral devices to properly measure your HR, but it is necessary with this and many other watches if you are serious about using HR training zones in order to improve your performance VIA HIIT workouts.
However, I would still recommend this if you're using it for steady state endurance activities. It works perfectly on long runs and the 5 HR zones are reflected accurately when compared against the apple watch or a chest strap after a 5 minute warm up. This is also the case on the bike.
I also think it's important and fair to mention that the watch itself is loaded with incredible features to track your activity so, if you can accept you'll need a chest strap for short HIIT and all cardio workouts to be safe, it may still be worth buying. The display is easy to see in sunlight, the HR sensor works on prolonged steady state physical activity aside from the beginning, and the battery life is godly. The watch also appears to be extremely durable. It's a tough call because the basics like HR measurement should be locked down by now with a company that is targeting a certain type of active individual but its HR data is far worse than the device made for a more mainstream audience.
Original: This is based on my experiences with the Garmin Fenix 5X Plus & Apple Watch Series 4. I took a close look at the heart rate monitoring abilities of each device by pairing a chest strap monitor to one device while using the optical sensor on my wrist on the other. I then swapped the chest strap to the other watch and repeated the test. I rode up a .25 mile hill at the same cadence twice before switching the chest HR monitor to the other device and repeating the same hill two more times. I found that the Garmin Fenix 5X Plus was slow to measure my heart rate and also inconsistent. The Apple Watch S4 was extremely accurate without any lag at any points. I would not recommend the Garmin Fenix 5X Plus because of its slow and sometimes inconsistent wrist based heart rate sensor.
