Simplicity Studio 5 has now been release and it is based on a much more recent version of Eclipse and CDT. It uses Eclipse 4.14 which was released in December 2019. One of the benefits of the newer version of Eclipse is better indexer performance. However semantic errors are still sometimes seen with Simplicity Studio 5 (but not always reproducible) with certain types of operations, such as when using typedef in header files.
If this occurs then try changing the default Indexer settings by disabling these settings:
[Index source files not included in the build]
[Index unused headers]
[Allow heuristic resolution of includes]
and then hit [Apply and Close].
Also it is still true that better indexer performance will be seen if fewer projects are open in the Simplicity IDE Project Explorer window because sometimes when the indexer updates it re-indexes all open projects.
Let us know if you still see indexer errors with the suggested settings. Silicon Labs is evaluating changing the default indexer settings in a future release.