How much performance benefit is there in adding a file (master page for example) via Module than code?
Getting the file from database (added via code) is slower than getting it from file system (adding it ghosted via Module) but how much?
How many % on average this is from a http request for example, considering that the file is retrieved each time during a request?
Does SharePoint always get the file from the database? Publishing at least has the object cache..
Getting the file from database (added via code) is slower than getting it from file system (adding it ghosted via Module) but how much?
Negligible.
How many % on average this is from a http request for example, considering that the file is retrieved each time during a request?
It's cached by SharePoint's code first, then ASP.NET's runtime, then SQL Server's various caches, then probably whatever disk based caching you have, so forget about it.
Does SharePoint always get the file from the database?
Files servered in virtual directories (those starting with an underscore) generally are not from the database, though chances are they use artefacts stored in the database.
SharePoint can be architected to easily handle 10s of thousands RPS, and is built around scaling horizontally. If you see a slowdown through request volume, you simply throw hardware at it after configuring your software (SQL Server, Request Management, etc) properly.
Adding files such as Masterpages via Modules are more manageable than by doing it through code.
Keep in mind that not all files are added to the databases, some files are added to the file system (e.g. in the /_layouts folder).
I don't have numbers on how much time you save doing one over the other, but I do know that with files such as Masterpages, Pagelayout, SitePages etc. which needs to be placed on the file system and only referenced in SharePoint, adding those through code is just silly.
