Any idea?

What version? What kind of files?

I don't see any slowdown on windows using Tk 804.030 and these files i just pbrushed

$ file 1.png 2.png 3.png
1.png; PC bitmap, Windows 3.x format, 32 x 32 x 24
2.png; PC bitmap, Windows 3.x format, 32 x 32 x 24
3.png; PC bitmap, Windows 3.x format, 32 x 32 x 24
[download]