Generating Charts with higher Resolution extreme slow

I use a Raspberry Pi 2 as openhab Server. If I look at my Charts via openhab Website, everything is fine. Chart generating takes one or two seconds. If I read the charts via Android App (App requests high resolution - depending on screen size), chart generating takes up to 30 seconds. Why it takes so much time? Is it possible to change the chart generator?

Thanks for your answers

I got a solution. I added the following line to my openhab.cfg:


Now the resolution is much smaller, but I get the charts very fast. Also with my droid.