If you find it can sometimes take 20–30 seconds to return the web part gallery so that you can drop a web part onto a web part page then this post is for you.
The problem may be because SharePoint is trying to contact the online web part gallery, however, if your machine is not connected to the internet, or the proxy is not configured properly, then it could take some time to actually work this out. This is the delay you are seeing.
You can easily turn off the web part gallery by following these simple steps:
1. Open SharePoint Central Administration (make sure you are on the Windows SharePoint Services page)
2. Click on “Configure Virtual Server”
3. Select the Virtual Server on which you are experiencing the delay
4. Click “Manage Security Settings for Web Part Pages”
5. Select the “Disabled” option next to “Online Web Part Gallery” and click OK.
You should now find it all working at lightning speed, I kinda wish we had shipped it this way.