The sluggishness that was previously an issue on Android devices is pretty much solved now.
In general it's a good idea to have the charts for neighbouring countries installed if you fly anywhere near the border. I think this example (Geneva) is almost certainly the most serious as the airspace extends 20nm into French territory.
Unfortunately there isn't a great answer to this. Although the French AIP does publish information on the Swiss airspace, we have found that it is often out of date and therefore inaccurate. The Swiss AIP is the official source of such airspace, but we do not refer to the Swiss AIP when creating our French chart.
It may be that a perfect solution will present itself at some point, but for the moment it's definitely best to have charts for neighbouring countries installed if you're flying within 20nm of the border.

I was recently caught out while flying Puimoisson to Troyes using Skydemon. The entire flight was over French territory, and the closest we got to the most western pointy bit of Switzerland was 15-20nm. It turns out that the Geneva Class C TMA from 5500ft upwards northeast of CBY VOR  (over French territory) is not shown unless the Swiss chart is loaded (sadly we were at 6000ft)! I had not installed this chart as I was under the impression that unecessary charts are one reason for sluggishness (Nexus 7).

Logically I can see how this happens and am not sure if anything can or even should be done about it. However, it was certainly not expected behaviour. Other than trial and error there doesn't seem any way to tell when I do or don't need a neighbouring country's chart. How far can airspace overlap neighbouring territory? Should I therefore always have Belgium, Germany, Switzerland and Italy all loaded if flying from the UK to southeast France just to be sure?

Am I mistaken regarding "extra" charts causing sluggishness? I have read in the forums about about dynamic chart loading so perhaps this is an old wife's tale?

Alan