The Photos app is nice but it can be a real system hog when it first starts up (especially on an older Mac). Every time you connect an iPhone, iPad or other photo containing devices the Photos app automatically starts. You can prevent the app from opening with the following command via Terminal (Applications > Utilities).
Note: The Terminal command is different than unchecking the “Open Photos for this device” option in the Photos app. That option has to be set for ever single device you connect.
Stop Photos From Opening Automatically (Copy and paste the following into Terminal)
defaults -currentHost write com.apple.ImageCapture disableHotPlug -bool trueRevert to Default Action (Copy and paste the following into Terminal)
defaults -currentHost delete com.apple.ImageCapture disableHotPlug