All of the below tests are done with the image at http://www.danamania.com/temp/largefile.jpg, a panorama created from 3 rather dirty snapshots out of a cheapie film camera, scanned & joined manually (forgive the distortion :) The image is about 946KB, and is a 5889x3711 image.
These main tests are done on a 1GHz G4 eMac with 640MB RAM. The GIMP and Photoshop tests on the eMac were both done after a fresh reboot, each app being the only thing running apart from the system.
For GIMP - Version 2.2 on OSX, Tile Cache set to 400, 500, 600, 800 and 1400MB - all had the same results within a second or two. Any less than 400MB tile cache with this image and the visible changes in both preview and the final 'OK' would hit a wall part way through the change and slow down drastically, with times for each change doubling or more.
Undo levels set to 1 and Maximum undo memory set to 128MB. GIMP shows the image taking 230MB at the bottom of the image window.
Adjust curves - 15 seconds to complete 'preview' across the entire image on each & every movement of the curve. It takes the same 15 seconds to 'apply' a change across the image if preview is off, after pressing 'OK'.
Color Balance - 16 seconds to preview across the entire image on each movement of an RGB/CMY slider. Stretches to 36 seconds on each preview if "Preserve Luminosity" is turned on. Same 16/37 seconds to complete the whole image if preview is turned off, after pressing 'OK' .
Levels - 16 seconds to preview across the entire image on each movement of the black/white/grey points, 16 seconds total to complete the whole image if preview is turned off, after pressing 'OK'.
Turning previews off makes things quicker by only requiring one 15ish-second pass over the image - however it also makes each tool effectively useless when trying to adjust by sight. A bit like working around having a dirty windscreen by driving with your eyes closed.
Photoshop 8.0 - Percent memory used set to 92% of available = 500MB on a fresh boot. Scratch disk is boot disk, and 'cache levels' left at 4.
Adjust curves - preview is instant, well under a second across the entire image area on each movement of the curve. After clicking OK takes one second to complete a curves change on the whole image. With preview off, also takes one full second to complete a curves change on the whole image.
Color Balance - preview is instant, perhaps 2-3 changes per second are reflected in the preview when moving the slider, and well under a second to complete the color balance after pressing OK. Same sub-second time when preview is off.
Levels - preview seems a bit faster than with color balance, almost smooth realtime transitions reflected in the preview. Pressing OK gives an instant change of levels across the image regardless of preview.
I also have an athlon 2600+ (1912MHz) running Debian and GIMP 2.2 that I had a chance to use earlier while it had 512MB RAM in it, and though I didn't get to use this exact test image, I tested with another I was working on at the time and had similar results to the 1GHz G4, just scaled in speed. That is - a color change in gimp that took 30 seconds on the 1GHz G4 would take say 18 seconds in gimp on the Athlon 2GHz.
In order to get past any background processing photoshop may have been doing, I also tried adjusting the curve in the curves dialog then quickly hit OK - this came out taking the same time as if I moved the curve and waited 10 seconds before clicking OK. Closing the image immediately afterwards was also instant, so there seemed to be no tricky background processing happening over the whole image area.
I hope this helps show the large image size type problems. I wouldn't expect two apps to give identical times for every process, but there's some big inefficiency in GIMP here that's making it more than ten times slower - especially considering that Photoshop on a Powermac 7300/200 (200MHz PPC604e with 140MB RAM) is not much different to GIMP on a 2GHz Athlon with 512MB for these particular operations on images at this particular size. (That's not a glib exaggeration either. Color Balance across the same panorama while I was constructing it on the 604e took about 10 seconds. It is running Photoshop 5.5). I stress "these particular operations on images at this particular size" because in other areas GIMP's speed is fine, however I'm often working on images at this size or larger.
Of course, if there are some memory/cache settings I'm missing out on to get GIMP down to the sub-second adjustments on images of this size, please tell me!.
dana -- http://www.danaquarium.com/
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