GPS Accuracy

No no wait. Look at the axis on the chart : it's time on the horizontal one. It should be the distance if you want to visually see something making sense for the average value. We want the time per km, not per minute. So if you switch to the same chart per km, it will look much more logical. The logic is still fine. But I'm not saying that the watch is measuring a reliable instant pace. Just that the chart and calculation are mathematically correct.

That's right, my mistake. average speed is not the average of the speeds over time :-)

Nevertheless I did some other checks, and even against distance, it doesn't work. Like 2464285, I quite often have an average pace on 1 km (autolap) better than any pace i've  seen during this lap. 

i plotted the instant speed (M/S, in red) from FIT file, available for every second, against distance (X axis) and the average speed given by the Fenix 6 for each kilometer (in green, data from FIT file as well).

we clearly see that for between km 5 and 6, i was almost always slower than the average speed for this kilometer, same for km 8 to 9, or 7 to 8. Only km 6 to 7 seems to be a little bit in the other side. 

and this is one of the most consistent pace / speed that i've seen since I started using this watch about 2 months ago. 

That is true and it was my first thought as well, I did a check in GC web and changed to distance on X axis and it looks better. BUT, the FIT file records speed at every second as m/s. It is obvious that the average you get in the end from the FIT file for pace differs quite a bit from the average the whole activity. 

I think that the reason is that for instant pace they have to calculate it every second based on GPS data but for average they can store GPS data over time and apply error correction which in the end will give a more accurate distance and a more accurate speed/pace.

I remember all threads about GPS accuracy and pace problems in the early days of Fenix3. It is exactly the same discussion today as it was back then. F3 got a bit better after a few GPS and FW updates but never to the point where it could be used for instant pace. I think that in the end everyone accepted, bought a foot pod or switched to another brand. To be honest there are few that is really good on instant GPS-based pace, Polar V800 and Suunto Ambit3 is the ones I know of.

I hope that F6 will do better but unfortunately I don't think that it will be good enough for those of you that really need instant pace info.

Just to reply, small pumptrack ride , GPS is fine, many autostop, but when i used in trail mode under wood, it's not so accurate...fw 4.30 gps only

track is most of the time about 5-10m off to the left

After update to firmware 5.00 no change. Again for a full kilometer current pace indication was 5:40-6: 30 and even 7: 20min / km never once below 5:30 and the average pace after kilometer 4:59. How it's possible? As I mentioned in the previous post, my 5-year-old suunto ambit 2 gives the current pace without deviations greater than 5 seconds (1 sec gps mode). So I don't understand why garmin engineers can't handle it.

Even if it isn't responding to pace changes immediately and sometimes is a bit off I've never seen the problems you have. I assume you use 1s recording (even though it shouldn't impact the current pace) and that 3D speed and 3D distance is turned off?
Can you post links to the activities you are referring to in Garmin Conenct?

It reminds me of the elevation calculation I was seeing on my previous Garmin watch :  i was going from 1000m at lowest to above 1600m at highest, and the total elevation was below 500m :)

And it was a long time ago. Since, nothing has changed, Garmin still provides plenty of new features instead of focusing on quality. Most of them are not very useful, some are really great and make Garmin the only choice in some cases. The pace is another typical example of this problem I think. The engineers are too busy working on the new features for the Garmin 7. 

Pity, I really love some of the features the Fenix are offering. I have just learned to live with the quality problems, it's the price to pay for the rest... 

Yesterday's training. Pace shown by watch and recorded between 3 and 4 km of my run. You can see that recorded pace was like 6:00 - 7:00 min/km and faster for some time. And again I know tand felt that real pace was like 5:15.

And pace after 1 km autolap was measeured correctly.

Link to activity:

https://connect.garmin.com/modern/activity/4326196963#

I have two different Garmins. I write quite honest about my disappointments:
Definitely for running, the Fenix 6 has very low gps for urban use, measuring typically short distances. Another big drawback is the useless pace. I don't know how serious a runner will use this F6 except under ideal conditions that are almost impossible in the city.
I have not yet seen a Garmin that counts the exact number of repetitions when using strength activity!
Still the Fenix 6 is not chinese garbage and is a bit strange for a $ 1,000 product.