Got map inefficient when using cairo

[kokeliang]

When using cairo to generate global map from submaps, the following code costed a lot of time. Would someone know how to get the global map more efficiently?

  CairoPaintSubmapSlices(1. / resolution, submaps, cr.get(),
                         [&cr](const SubmapSlice& submap_slice) {
                           cairo_set_source_surface(
                               cr.get(), submap_slice.surface.get(), 0., 0.);
                           cairo_paint(cr.get());
                         });