No guarantee or warranty expressed or implied. Observations are based purely on my own experience. All testing and experimentation is strictly at your own risk.
There is a file called reimport.lst in the skin directory. SKS uses it to keep track of the images you have imported or edited from within SKS and monitor them to see if they've been updated.
Just to see if reimport.lst is causing your problem:
After SKS has become noticeably slow, close the skin in SKS.
Within Windows Explorer, copy the folder of your WB skin to back it up, "Just in case".
Within Windows Explorer while in the skin's folder, move the file "reimport.lst" to another backup directory (yes, redundant, but...)
Open the skin back up in SKS, go back to the sections you were in and see if the sluggishness is gone. Don't do any image editing or image importing at this point.
Close the skin.
Copy the file "reimport.lst" back from its backup directory into your skin's folder. You are now back where you started from.
Gory Details you should read:
If you leave the file out or delete it SKS will have no record of your importing or editing up to that point and it will not monitor changes to those images. It's a option though if it fixes your problem and you're ok with abandoning your import and edit history. This means the "Force image reimport" link will no longer import any source images that you changed since moving or deleting reimport.lst until you import them again. If you change any of the source images directly you would need to import them individually using the "Import" or "Quick import file" options. So, you would want to move reimport.lst when you're reasonably comfortable that you're not going to want to go back and change a lot of images. It would also be better to "move" it and keep it for a bit rather than "delete" it, in case you want to move it back and reload for some reason.
It also means that, until you import a specific image again, when you use the "Edit image" button to edit that image the source image itself won't be edited, .bmp's will be edited and saved directly within the skin folder, and SKS will convert .tga's from the skin's directory to .pngs and [open them from] / [save them to] the "TempPngs" folder within the skin's directory. Your original source image would not be modified. After deleting reimport.lst SKS has no record of any existing .png's in "TempPngs", so it will also overwrite a .png in that folder if it was created before the reimport.lst file was deleted and you decide to edit the image again - move any png's out of that "TempPngs" folder if you want to protect them.
After moving or deleting the reimport.lst file, SKS will start up a new reimport.lst tracking imports and edits going forward when you import or edit an image. It will not include any of the previous references, only the images imported and edited since the new reimport.lst was started up. Source images for the images imported and referenced in the new reimport.lst will be modified directly when "Editing", "Force image reimport" will import any changed images referenced in the new list.
I hope all that makes sense and helps.