We have a 3rd party developed Lightning App where most of the components don't render until after a 50 second delay on the first use of the day. But you can e.g. open the Salesforce Sales app first and that opens quickly: the problem seems to be with this specific custom Lightning app. Though we do see a similar big delay for this Salesforce Labs Package Visualizer App.
I have not seen this described anywhere, so that makes me think that it may be a problem in the app, but there is nothing obviously wrong with the app that I can see (apart from it using way too many of its own LWCs).
So before I do a lot more measuring/testing/logging, is a big startup delay on a custom Lightning app using many custom LWCs normal?
This Why is my Lightning App inside a Managed Package extremly slow during its bootstrap phase (but not in the DEV org)? may be the same problem.
The call that blocks (for 48 seconds) is the first call of this:
with an immediately following call to the same endpoint taking 700ms.
This seems like a similar "org warmup" effect to what is seen for Apex code, where the first access to an org takes a few seconds extra while the Apex code is compiled and cached. Is the platform pushing all the LWCs through (slow'ish) tools that generate the single .js file per LWC and caching that result?
(We have now created a Saesforce case for this question too.)