P: Slow preview in full screen (F)

It has become painfully slow in Mac Mojave when I press “F” to view previews in Full screen. Takes about two secs to switch between pics, even if previews already exist. It goes instantly to switch between pics in Loupe mode. Tried turning graphic processor on and off, without significant difference. 
(On an 27" iMac 4 GHz Intel Core i7 24 GB)