To get an accurate position on where you are on the track at a given point in time, you need to get numerous position reports/second because of the speeds involved.
818xt is capable of providing position reports at 10Hz--10 reports/ second. I usually have mine locked at 5hz--5 reports/second because of limitations on my phone. Your generic android phone GPS usually runs at 1Hz--1 position report/second.
At 60mph, you're covering 88 feet/second; with a GPS running 1 position report/second going down the interstate this is no big deal--accurate enough for your purposes. On a race track at twice that speed--say 180'/second--your position reports can be off by nearly 60 yards at those speeds, which will compromise your track data as far as how fast you were going at what point on the track.
If you look at some youtube posters using an iPhone/harry's lap timer--assuming they are just using the phone's internal gps--this lag in speed vs. the video is very apparent; you know the guy is accelerating but the indicated speed seems to move slowly--then boom, it's +20mph faster than it was a second ago.
So, to put it simply, because of the speeds on the track, you need something faster than 1hz reporting if you want accurate speeds at a given location. 10 Hz would be great but a 5hz gps device seems to work fine for me.
Hope this helps; BTW Qstarz also sells a complete data logger--does the function of the RC software--besides records position reports-- for about $170.
Here's a link to Qstarz:
Shop QSTARZ | QSTARZ Q1000eX GPS Lap Timer