iOS devices operated in warm / hot environments while active, run into overheating problems from time to time. This post covers a few things you can prepare to work around this issue:
1. Reduce energy consumption as this directly relates to heat developed
2. Shade your iOS device from direct sun
3. Place you device in a cool place
The first point is pretty obvious. Here are the worst energy consuming components of your iOS device - from strong to low impact: Video capturing, wifi operation, bright screen, cell operation, Bluetooth operation.
The video capturing process heats the iOS device quite a bit, just record a few minutes and touch the area around your lens - it gets pretty hot. There is not too much you can do about this as long as you need a video recording. However, I want to repeat my recommendation from http://www.gps-laptimer.de/Video.html
: "… my personal recommendation is to not go for video recording all the time. You miss a lot of fun and track focus when fiddling around with video all the time. So my suggestion is to train the first half of a track day using standard data/time recording and then video record a trained session only. This give you enough time for a great video."
Wifi operation should be turned off generally. Only exception is you use one of the wifi connected external accessories like OBD dongles etc.
Screen brightness can be reduced to approximately 80% without any impact on readability. I'm not sure the 20% make any difference in power consumption, but it is worth a try. In case you do not watch LapTimer while driving, you may consider reducing it far below 80%.
Cell operation is not significant as long as you do not use online access. Standard operation of LapTimer is sending position and lap time data while actively lapping. You can turn this feature off using LapTimer's setting "Online Racing / Publish" and switching to to "Never". Turning it off will not allow your friends to view you on track, see yellow flags, or get your time into the Hall of Fame however.
Bluetooth operation is the least significant component. Please treat it like Wifi operation and turn it off as long as you do not have an external sensor connected.
Next point is direct sun. You can overheat your iOS device by placing it into direct sun easily. That applies even using an app not utilizing your device's components as intensively as LapTimer. A typical beach problem
So try to place your iPhone in a shadowed position, that helps a lot.
The last point is the most powerful approach. I personally never ever ran into any overheating problem with any of my devices - even with all features above turned on. Reason is I have my iPhone placed directly into one of my car's air fan streams. Please check this option first, in case you have the opportunity to place your iPhone e.g. like in the picture below, you will be on the save side.