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• Obviously PEP8. Especially whitespace and abbrevations bug me: grn is so not more readable than green.
• Some variables are unused, e.g. smbytefunc and tobytefunc.
• Instead of clamp you can use numpy.clip.
• Loops with range(len(...)) are probably better written with enumerate instead. If you have a list of images you can also just iterate over the contents, instead of using the indexes from range(number_of_images).
• It is less error-prone to keep just one list of images/widgets/... than to keep the number of things as a separate variable; e.g. both nimgs and smid is more likely to get out of sync with each other. If you want a shortcut, maybe use a property for nimgs instead.
• Using both Tk and Qt is not nice; (if you're already using Qt, than) try to minimise the number of (additional) dependencies in your application. I found this question for the screen size and it seems to work fine for this setup.

Let's start with downsamp. I guessed that the mean calls at the end where a problem, because mean comes up in the stats as well; searching for that gives me this Stackoverflow answer, which adapted to the code gives a bit of a boost:

• Obviously PEP8. Especially whitespace and abbrevations bug me: grn is so not more readable than green.
• Some variables are unused, e.g. smbytefunc and tobytefunc.
• Instead of clamp you can use numpy.clip.
• Loops with range(len(...)) are probably better written with enumerate instead. If you have a list of images you can also just iterate over the contents, instead of using the indexes from range(number_of_images).
• It is less error-prone to keep just one list of images/widgets/... than to keep the number of things as a separate variable; e.g. both nimgs and smid is more likely to get out of sync with each other. If you want a shortcut, maybe use a property for nimgs instead.
• Using both Tk and Qt is not nice; (if you're already using Qt, than) try to minimise the number of (additional) dependencies in your application. I found this question for the screen size and it seems to work fine for this setup.

Let's start with downsamp. I guessed that the mean calls at the end where a problem, because mean comes up in the stats as well; searching for that gives me this Stackoverflow answer, which adapted to the code gives a bit of a boost:

• Obviously PEP8. Especially whitespace and abbrevations bug me: grn is so not more readable than green.
• Instead of clamp you can use numpy.clip.
• Loops with range(len(...)) are probably better written with enumerate instead. If you have a list of images you can also just iterate over the contents, instead of using the indexes from range(number_of_images).
• It is less error-prone to keep just one list of images/widgets/... than to keep the number of things as a separate variable; e.g. both nimgs and smid is more likely to get out of sync with each other. If you want a shortcut, maybe use a property for nimgs instead.
• Using both Tk and Qt is not nice; (if you're already using Qt, than) try to minimise the number of (additional) dependencies in your application.

• Obviously PEP8. Especially whitespace and abbrevations bug me: grn is so not more readable than green.
• Some variables are unused, e.g. smbytefunc and tobytefunc.
• Instead of clamp you can use numpy.clip.
• Loops with range(len(...)) are probably better written with enumerate instead. If you have a list of images you can also just iterate over the contents, instead of using the indexes from range(number_of_images).
• It is less error-prone to keep just one list of images/widgets/... than to keep the number of things as a separate variable; e.g. both nimgs and smid is more likely to get out of sync with each other. If you want a shortcut, maybe use a property for nimgs instead.
• Using both Tk and Qt is not nice; (if you're already using Qt, than) try to minimise the number of (additional) dependencies in your application. I found this question for the screen size and it seems to work fine for this setup.

Oh and now that I think of it: I'd rather (up/down)scale the images once when the setting changes and then only index into the already scaled version instead of doing that on every mouse move.