Why is the recorded path off-road / the speed incorrect at places / lap timing missing some laps?

The position and speed reported by GPS is not always correct. It’s normal to be off-road, and having incorrect speed displayed occasionally. If your recorded path goes off-road by a lot near Start/Finish line, it will not be registered and will cause two laps to be “joined” as one.

The data accuracy is about the GPS satellite reception; the better the reception (and the receiver itself) is, the more accurate the logged data is. There’s also quite a few circuits that are really difficult for the GPS receivers, such as the Nordschleife in Germany, where are trees and hills blocking the view to horizon and the satellites. You should expect at least some data accuracy problems in places like that.

The GPS reception can be improved by careful placement of the GPS receiver. If you’re using the internal GPS on your phone, the first step would be to install the phone to a phone holder, so that there is clear view to sky from the phone. The internal GPS does not work properly, or not at all, if the phone is in your pocket, passenger seat or glove compartment.

Second thing you might try, especially if the first step does not improve the situation, would be to purchase an external GPS. See the recommended devices in this FAQ: “Which GPS receiver should I buy?”.