If you have the tab s 8.4 try installing google now launcher or nova. Then try swiping between the desktop pages in both portrait and landscape modes.
My tab s 8.4 lags considerably in landscape but not at all in portrait. The Touchwiz launcher has no such problem. It seems to have something to do with the wallpaper so try different static wallpapers.
The 10.5 has no such issue and zips along beautifully in both portrait and landscape. It seems to be clearly a software glitch since touchwiz has no issue and with some wallpapers the problem gets reversed and portrait lags.