Broken cache causes massive performance losses

[Paul Kallnbach]

- Inkscape rev 12851, Linux (Debian 8)

program behaviour indicates that the statement in file "drawing-item.cpp" line 586 - 588 is called for every
object every time the canvas is updated, resulting in massive performance losses for files containing many objects.

    if (cl) {
        _cache = new DrawingCache(*cl);
    }

outcommenting these lines immediately increases the programs responsiveness by multiple factors, esp. on high zoom levels.

not really related but maybe useful: increasing inkscapes limit for completing a complete redraw from 1ms to 100ms in "sp-canvas.cpp" additionally helps to create a much smoother user experience:

    if (elapsed > 100000)

[Liam P. White (liampwhite)]

Well. This just allowed me to work on a massive drawing.