What's weird is that it uses to work fine for you. Have the pictures gotten bigger lately?
I used to use XnView. As far as I know it should be simple and efficient.
The thing is, if there was no change in the image size, format or anything else, there should be no reason why it would be slower. I think that's the real problem. Try scanning your computer for malware. The way to do this is by running a bunch of programs, although a few would be enough. If you want to be thorough:
2- Zemana portable
7- Optionally reset your browser to default settings
8- Optionally run EEK
9- Open CMD as Administrator and run
This should take a while but it should leave your computer clean as a whistle.