I personally turn off the RetroPie splashscreen and turn on the intro video for Attract-Mode when booting directly into Attract-Mode. It looks cooler, boots faster and it's controlled by Attract-Mode so you don't need to worry about any overlap and can also cancel it by pressing a button. If you want to use this feature you can put your video in /home/pi/.attract/intro/ and change the Attract-Mode settings using Configure-->Intro. To turn off the RetroPie splashscreen you can use the RetroPie setup tool.
If you want to delay Attract-Mode when booting you can add a "sleep 5" line into the /opt/retropie/configs/all/autostart.sh (\\retropie\configs\all\autostart.sh using the samba share) file.
Add
sleep 5
on the line above
attract
You can change the number of seconds after "sleep" if needed.
P.S. If you go for the Attract-Mode intro video and disable the RetroPie splashscreen option, you can speed up booting even more by using the raspi-config to disable the OS waiting for the network when booting. You'll still connect to the network if it's there but will boot faster (especially if the network isn't there, like when you bring your Pi to someone else's house).