plot save time scales with plot complexity

[BobM]

I'm a relatively new user, running octave 3.6.1 on a Windows7 machine. 

After making some fft plots with large amounts of data (~ 150000 data
points) I found that "saveas(gcf, 'xxx.png') would take an exceptionally
long time (~ 1360 seconds)

as an experiment I tried the following
t = (1:1000);
plot(t)
tic(),print 'test.png' -dpng,toc()
et ~ .76 seconds
for t = (1:10000);
et ~ .85 sec
for t = (1:100000)
et ~ 18 sec

If I make the plot more complicated, but with the same number of points 
t = (1:100000)
b = sin(2*pi*t/1000)
plot(t,b)
et to save ~ 19 sec
b = sin(2*pi*t/100) time to save as png goes to ~ 50 sec.

It seems like the time to save the plot as an image is scaling with the
number of points and the complexity of compressing the image, but this seems
a bit unreasonable. It seems to me that the image resolution would should
limit the amount of information that needed to be transferred.

Is there some setting in octave\gnuplot that controls the degree of sampling
that happens between the raw data and the display or image output that needs
to be adjusted?

Thanks in advance for any help, I apologize if the question is poorly posed.



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