1. It won't change the default selection, but you can prevent users from saving Attachments everywhere except your wiki_attachments area. Only areas where attachments are allowed will appear in the "Save to" drop-down. For areas that should not accept attachments, edit the area in the Area Manager, and find the section for "Allowed Content Types in this Area".
2. There is a delay for the File Browser which can be improved, because it currently waits for the contents of the browser to finish loading before opening. Since there are many images out of the scroll view, we don't have to wait for all of them, and that is definitely an improvement we can make.
When you click "New Attachment" though, there is a separate delay that is due to 2 sub-requests: 1 for the create-new-page script and the second from that script to the server to generate the correct form. I'm not sure much can be done to improve this delay, as it depends mainly on your server response times, but maybe we can show a loading icon or something so the user knows something is happening after they clicked.