So, feel kind of like I'm talking to myself now, but I believe I have figured out the issue.
After noticing the lag on an OpenPHT client installed on the Plex server computer, I figured it did not have anything to do with the laptop running the client in the theater, and it is most likely not the network.
I then started looking at the other views displaying posters, and looking for lag there. I noticed that the carousel view does not exhibit lag when opening it's posters, unlike the pre-play screen.
A comparison of the controls that load the poster, yielded one material difference:
The Carousel view used - texture background="false">$VAR[ListItemPoster]
The Preplay view used - texture background="false">$INFO[ListItem.Art(bigThumb)]</texture
I'm not a kodi/skinning expert, but it looks like the preplay view is using some kind of array to pull the poster, while the Carousel is directly referencing the poster already selected from list? Must be some lag in reading the array and selecting the right texture, is my guess.
What I do know is that when I took the texture background line from the Carousel view and swapped it into the preplay view- the poster loads immediately. No lag at all!
I have selected movies from the other views, the home screen's on deck and they all seem to work fine. So it doesn't look like I broke anything else in making this change.