Welcome to Android Central! Are you asking how to erase the videos that are in your Google+ Photos in the cloud, or how to erase them locally on your device? Which phone do you have?
Video files shouldn't slow the phone down. They might cause low memory alerts if you're running out of space, but they should impact performance. If they are backing up over the web, that might slow things down--you can turn off the Auto-Backup for videos specifically in the settings. Open Photos, tap Menu>Settings>Auto Backup, and turn off the Auto Backup Videos setting.