Is there any provider that feeds results from all project files or at least all open files?
it will be huge performance hit.
Why is that?
You don’t need to parse all files all the time. You can do it once you open a file, or when you open a folder.
The default provider has an option to include completions from all the open buffers:
AFAIK the symbol provider may be able to provide completion for symbols from the whole project when using CTAGS.
Built in auto-complete.