Can you tell us please more about your migration from iPhoto to Photos? This may help to find out, what the problem is.
- Which version of macOS X do you have installed?
- What are your settings for iCloud in "Photos > Preferences > iCloud"?
- What are your settings in "System Preferences > iCloud > Photos >Options"?
- is your Mac connected to Wi-fi or are you using a cellular hotspot?
- Is your Photos Library in the Pictures folder or are keeping it on an external drive?
- How long ago did you enable iCloud Photos? With a library of 260GB the initial upload may take roughly two weeks, even with a fast Wi-Fi connection and keeping your Mac and Photos up and running all around the clock.
- Have you checked your iCloud Photos Library on your iCloud web page at www.icloud.com? Sign in with your iCloud AppleID, open the Photos.app, then see, which photos and videos have already been uploaded.
- In addition to Lindsay's advice - you may want to check the status bar below the Photos view. It will tell you, if Photos is still uploading to iCloud, or if there are items, that cannot be uploaded. See this Help page: About the status bar in Photos for macOS - Apple Support
If the status is showing that Photos is still uploading, you just have to be patient. Are the numbers of items to upload still changing, albeit slowly? Then do not interrupt the upload let it run. You may want to change the System preferences of he Energy Saver, so your Mac will not go to sleep while the upload is happening.
- If there items listed as "unable to upload" you have to remove them from the library. This is described also here: About the status bar in Photos for macOS - Apple Support
- If there are items "On this Mac only", you are having referenced images in your library, that need consolidating into the library. Photos, that have not been imported to your iPhoto Library and are stored outside the library, will also be referenced in Photos and cannot be used with iCloud Photos.