When you import those images, you should drag and drop to a proper size. It will help not to have all the image on top each other.

To select them you can use Object Docker, and it will be easy.

You may have few RAM and that is why your system slow down. If you don't need Undo List, disable it in Tool/Options/Memory.

Check how much resolution you have in your new document where you are going to be importing all the images. You may have more resolution than necessary.

About the transparent tool. You should practice with small image independenly, then go for your big project.

Regards