I guess I was a little confusing in my previous post.
I was originally using mesh. When I read your suggestion of modifying the "__gnuplot_raw__" lines, I read mesh.m and didn't fid such lines in it - mesh.m
uses __gnuplot_set__. So I tried modifying mesh.m to use __gnuplot_set__ nohidden3d, but the resulting plot was also very slow (like the plot done with the unmodified mesh).
Then I discovered that meshc.m uses __gnuplot_raw__. I tried
meshc.m without modifications, and got the same slow plot. Then I modified meshc.m as you suggested, and it got a little faster when using AquaTerm. Then, at last, I changed my terminal to x11 and the modified meshc gave me a much faster plot.
That's why I guess the problem lies in AquaTerm, and not in gnuplot.