I really hate this. Outlook is running fine otherwise, however, when I try to attach a file to an email the process slows to a crawl. Frustrating!!
First, try to run Outlook in Safe Mode. Click Start and then Run, (Win7), or use Cortana, (Win10), and type Outlook /Safe and hit Enter. If necessary, choose your profile, (usually Outlook), and click Ok. Now try to attach a file in this mode. If it works fine then you will need to disable add-ins.
To disable add-ins, restart Outlook in normal mode. Then click File, then Options. Then click the Trust Center and look for your add-ins. Click Com add-ins. Select the check boxes to clear the add-ins and try attaching the file again.