OS X Mavericks screensaver changes pictures too quickly: How do I slow it down?
Previous answer still works, and also for Yosemite/El Capitan/Sierra with the caveat that System Integrity Protection (SIP) now needs to be disabled first.


Verify current slide duration:

sudo /usr/libexec/PlistBuddy -c "print ':JustASlide:mainDuration'" /System/Library/PrivateFrameworks/Slideshows.framework/Versions/A/Resources/Content/EffectDescriptions.plist


The system default is 3.
Reboot and hold Cmd+R to go into recovery. In the menu select Utilities > Terminal and disable SIP with the following command:

csrutil disable

Reboot again and set slide duration to the desired value in seconds:

sudo /usr/libexec/PlistBuddy -c "set ':JustASlide:mainDuration' 10" /System/Library/PrivateFrameworks/Slideshows.framework/Versions/A/Resources/Content/EffectDescriptions.plist

Reboot one more time into recovery (Cmd+R) and reenable SIP for good measure

csrutil enable