A pop up menu formatted to show the second value with the "override data formatting with value list" option enabled is essentially putting a field from a related table occurrence on your layout or an unstored calculation as each time that you scroll it into view, the system has to do a query to find and display that value from the value list table's table occurrence.
That makes sense. Although, I'm not exactly sure what the 'override data formatting with value list' option does in the first place. I'll have to go read up on that. Hmmm...helps somewhat, but still seems vague (from FM Help):
Note: For closed pop-up menus, to display the value from the value list and ignore data formatting, select Override data formatting with value list. This is useful when, for example, the two fields are different field types.
I did some more testing after reading your note and it appears that even just putting the simple related table's "fieldID" on the layout causes similar hitching in the scrolling. My original thought was that it had more to do with the value list associated with the picker and not just being related data. I should have tested that first. (End of day rush, I guess.)
Although, it does seem that when the field formatted with a drop down is on the layout, even if the field is a local field but the value list is from a related field, just having it sit there incurs the 'related data' penalty. That is, even though the related data isn't showing, it appears to be evaluating the value list and causing the delay. For example, I'm showing the 'CategoryName' field from TableA on the layout; my value list is two parts - TableB::ID and TableB::Name. There's a scripted process that copies the 'TableB::Name' field back into 'TableA::CategoryName' field when the user makes a selection. Having a drop down showing 'TableA::CategoryName' on the layout as a pop-up menu incurs the same penalties (it seems) as it would if I showed the 'TableB::Name' field directly.
My biggest problem still seems to be images - whether from a local container field or a related Images table, the loading of images causes very noticeable usability hit.
It would be fine if this loading process didn't interrupt the rest of the UI rendering (specifically scrolling in my case) but it does seem to have a higher priority than the general window controls and interrupts the scrolling ability. So even though I want to scroll down to page 12, the loading of the images makes it pause slightly as it scrolls by them on each page in between. There's a product suggestion for the developers, I suppose.
Create thumbnails the size you need for a layout. These load a lot faster since they are often 1/5th or 1/10th or less than the original image.
Image thumbnail 320
Image thumbnail 1024
If you need to show the full image say on a 27" imac screen, then create a layout just for the Image and use that via a clickable.
That's a good idea - I had done this before on another layout I was trying to improve, but hadn't applied that yet to this one. I'll give it a shot.
It's perhaps a bit clunky and not as elegant, but the way I got around this performance hit for the drop-down + value list issue was to show the local field (as I mentioned), but instead of a drop-down field I created a pop-over with a portal in it. Then the portal has a full-row button that fires the script to do the 'selecting' of an entry.
It's perhaps a bit heavy handed, but it does improve the performance of this layout.
Not sure that I would call that "heavy handed". A popover with selection portal can do many things not even close to possible with a drop down list or pop up menu alone, such as showing data from more than two fields, sorting the records on a field not visible or in an order other than ascending to name several examples.
True, it has a lot of possible flexibility. But it does require a fair bit more setup in terms of layout objects, styling, scripts, etc. I put a little radio-style button to indicate which one was currently chosen, too, so more calculations there.