Why GPS in 5 plus is SO bad (SOLVED)

Alan, that is because your track is 400m long, this is extensively discussed above. Try to run 200m track and you will be surprised. 

I discovered another GPS issue yesterday during long run in the forest. Although the track and pace at the end looks good, it seems there is an algorithm that corrects GPS availability signal _during_ the run. If the sky is covered by large trees then current pace indeed tends to be underestimated but corrected upon finishing laps. For example, I specifically gazed watch for specific kilometer and they showed 6.00 pace most of the time (definitely not more) but upon lap (km) the average pace turned out to be 5.30 (that is actually closer to reality). There is no such effect with open sky. 

They might correct the distance but not the position! You can see that my watch very well tracked the lower curve and across the grass with the sharp edges, this is less than 200m!
If your theory would be true it would have corrected my shift to the outer lane. I did much more track trainings with my Fenix 1 and there it can be seen if I run the slower parts on the outer lane.
If you have constant issues with your watch at different places, contact Garmin and send it for repair! It should deliver good GPS-Tracking!

To your second issue:
This is true and a madder of the measuring principle. The slower you are the less accurate is the current pace, that's why it works well for cycling and is barely useful for running and it's better under open sky than under trees. The Garmin foodpod helps a lot if you are running at a constant pace. For intervals or track training, when you change both frequency and length of the steps, it is also not helpful. A Stryd should be better, but I never had one.

Alan, that makes sense what you said but nevertheless that is fact. Small circles make the difference despite of fact that F5+ handles sharp turns well. I pretty sure that is software flaw because that is single device on the market with such problem. All older Garmin devices had no this problem. And it is not a problem of my specific watch, just read the whole topic. Or - just run few 200m circles - and I promise, you will be surprised!  

For the second issue, I'm not sure but probably you right and it is common problem for any device. My old Vivoactive HR sometimes _decreased_ already run distance during the run that drove me mad and it was one of the reasons to replace them. F5+ at least doesn't do that. I don't know how competitors like Polar perform in this aspect, never owe them. Smarphone-based Strava, however, seems never had this issue. 

I haven't your issue.

This is an example based more than 30 laps on 400mt track

Fenix 5s Plus - GPS+Glonass

The issue is for 200m tracks or less. 400m tracks do not have problem, that was already discussed above. 

Finally, I solved the problem - I bought the Stryd. Precise and instant pace and distance are just awesome (I'm not belong to Stryd but really :) Their "power" concept seems also great.

Just FYI, I have contacted Garmin regarding the issue and they authorize watch replacement. According to Garmin, it's "clearly malfunction". However, I don't think so and finally decided do not waste a time with replacement. I have okay GPS performance everywhere except short circle tracks and I have excellent pace/distance metrics with Stryd.

Nik_Riaz said:

I solved the problem - I bought the Stryd.

Good choice!

I've been using Stryd for more than a year now, and I think I can guarantee you will not regret!