If you use 'Optimize for clicks' in your Ad Rotation settings, your ad testing can take significantly longer because Google may favor one ad over another (e.g., based on CTR data) and show it most of the time. Hence, it's recommended to use 'Rotate indefinitely' for your campaigns. You'll receive an alert whenever a campaign is not set to this setting (see Audit Alerts / Campaigns on your Adalysis Dashboard).  Click on the alert to view the list of affected campaigns. 

You can change this setting directly in Google Ads as described here.