Paul Debevec is the Godfather of environment mapping techniques, such as IBL.

He wrote:

Quote - It may be good to downsample the environment image first since the convolution calculations can take quite a long time.

The purpose of that parameter is for you to DOWNsample your image, not keep it high res or, shudder, to up sample it.

http://ict.debevec.org/~debevec/HDRShop/tutorial/tutorial6.html

Quote - Resize the image. Click on Image in the menu bar, then Size from the Image menu then Half Size from the Size menu. Repeat this several times until the image has been resized to a point where HDRshop can calculate the convolution in a reasonable amount if time. In this example the image has been resized to 128x64 and the convolution calculation takes about 96 seconds.

Things are faster now, but he was dealing with 96 seconds!!!!

Most of the pros use 128 or 256 pixel IBL.

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