You can test on sites and networks you use to see what you think. Private tabs do not use the cache shared by regular tabs, so if you have a slow connection, pulling files from the cache for a site you visit often can be faster than re-downloading in a private tab. By default, private tabs use Tracking Protection. Sometimes tracking protection blocks a script that a site expects and there is a delay while the site figures out that the external script is not going to load. Otherwise, when comparing first time loads in regular and private tabs -- cache not a factor -- the private tab often loads faster due to not having to deal with the tracking content.