[racket] big-bang is slow to render on screen?

I have a 2htdp/universe program that used to run fast enough a few months
ago, but now it is very slow and not usable.
The slowness seems to be because of the on-screen rendering, and not
because of the generation of the image.

Here is a stripped-down version that shows this behavior:
https://gist.github.com/Metaxal/11142941

In the following log, you see that the `on-key` events are very close one
to the other (in milliseconds after the beginning of the program), but the
corresponding `to-draw` events are separated by more than a second, even
though generating the image (cpu time) takes almost no time:

on-key a at 6906
on-key u at 6912
on-key i at 6912
on-key e at 6913
to-draw at 6913
to-draw: cpu time: 4 real time: 3 gc time: 0
to-draw at 8598
to-draw: cpu time: 4 real time: 2 gc time: 0
to-draw at 11948
to-draw: cpu time: 4 real time: 2 gc time: 0
to-draw at 13631
to-draw: cpu time: 0 real time: 2 gc time: 0
to-draw at 161839
to-draw: cpu time: 4 real time: 9 gc time: 0

During those long seconds, Xorg is almost at 100% cpu.

However, using an empty scene instead of an image is fast.
The time also depends on the size of the grid.

I'm using Ubuntu 12.04 64bits.
I have tried to replicate the behavior on older versions of racket (5.3.1
and 5.90.0.9) but it's the same. So maybe the problem is not on Racket's
side but something has changed in Ubuntu?

Does anyone else see the same behavior, either on the same platform or on a
different one?

Thanks,
Laurent
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