Subplot demo performance

[Konstantinos Poulios]

After recent changes in the calculation of axes
position/outerposition, Ben observed very long rendering delay for the
1st subplot demo. Since I cannot reproduce this problem I start this
new thread in order to ask for some testing from others, so that we
can narrow down the cause for this regression.

@Ben, since you are the one who can reproduce this issue I would
suggest to start with some more basic tests than the subplot demo, so
e.g. you can try:

1. Drawing a grid of 3x3 plot with no data and axes labels/titles.

2. Add some data to the plots

3. Add titles

4. Add axes labels

Depending on the delay you observe in these tests we will be able to
narrow down the problem a bit.

BR

Kostas

P.S. Once we resolve this I will open a new thread for the subplot layout issue.