Unstored calculation fields.
Excessive conditional formatting.
Lots of fields/portals on a layout that is showing lots of records
I will go back and tweak the settings(iteratively) until the end-user experience is smooth - while retaining the formatting which makes the user interface looks slick.
A good way to test is to selectively remove elements from your layout until you find what is causing the slowdown.
I try to 'binary chop' it because that is the fastest way, remove half the layout and see if the problem is fixed, keep removing halves until you find the issue.
It could also be a combination of unstored calculation that is causing the issue.
If the layout is slow over a WAN you can also use the network tab of activity monitor on OSX, or 'Glasswire' on windows to evaluate the network traffic.
Pay attention to elements you may have copied from other layouts which contain conditional formatting, calculated tooltips or calculated Hide conditions that reference fields that are not related to the current layout. This has caught me once or twice.
Table Width — the number of fields in the base table of the TO for each layout.
For instance, you have two identical layouts, each showing only one indexed number from its base table, but one of the base tables has 30 fields, while the other layout's bast table has 600 fields...
The layout based on yhe 30-field table will scroll and refresh MUCH faster.
It has to do with record caching to the client.
Thanks to all for some powerful tips that sound battle-tested- I will be benefit immediately from these!
List view layouts are very costly. Try to avoid them. Filemaker Server sends down lots of records trying to anticipate your scrolling down the list.