It's not the app you should be worried about, the app just logs the data fed to it from the GPS module, if that data is accurate, then your times are also accurate. Standard GPS modules in smartphones are only 1Hz which equals to just 1 value per second (although they use some predictive algorithm to improve accuracy) and clearly for such use it's not accurate enough.
An easy thing you can do that offers much better accuracy is to buy an external GPS module like:
http://gps.dualav.com/explore-by-product/xgps160/ that works at 10Hz and a lap timing app of your choice (RaceChrono Pro, Harry's Lap Timer etc.). It's a pretty accurate setup at a fraction of a cost of a p-box with much improved accuracy over a smartphone. I did that switch and then compared graphs of my driving at the track. In the smartphone log It seemed like I was zig-zaging everywhere and my speed values were weird. After I switched to XGPS160 + RaceChrono the telemetry looks much smoother and "normal". Total cost was around 110 eur for the GPS and then 5 or 8 eur for the Pro version of RaceChrono app which you can use to measure any acceleration you want (not just lap times).
In a sense, it's my own pretty accurate road dyno as I 've used it to compare different cars on the same piece of road or just trying to quantify the benefit of different modifications.
