In a new Knowledge Base entry, Apple reminds users that photos are backed up each time you do a sync, even if there are no changes to the Camera Roll. The result can be a sluggish backup or restore.
If you already have the photos copied into iPhoto, you're good to erase them from the Camera Roll. Apple has some advice for getting the pictures back to your iPhone if you need to:
2. Open iTunes.
3. Select the iPhone in the Devices list in the left column.
4. Click the Photos tab.
5. Ensure that "Sync photos from" is enabled and choose the appropriate source from the adjacent pop-up menu. For example, if you imported your photos into an application (like iPhoto), you would select that application in the list. If you imported your photos to a folder (using Image Capture, Scanner and Camera Wizard, or Windows Photo Gallery), you would select Choose Folder and then the folder in question.
6. Click Apply or Sync.
The article details procedures for Macs, Windows XP, Vista and Windows 7.
There are other things that can slow down backups. I've seen them be very slow one day and quite snappy the next. Explanations can sometimes be hard to come by, but regular backups will keep the times down. Also, if you've added a lot of new apps, your first backup after adding them can be pretty long.
For more details on the issues with a full Camera Roll check the Knowledge Base article, and don't let those photos on your phone get out of control.