Download the process monitor from the SysInternals suite and take a look at the frequency of registry queries. Reading and particularly modifying text files at that rate would be a significant burden on the system.
Since the registry was introduced, both windows and applications access the registry as if it were a no-cost operation (in other words, far more often than is actually necessary or advisable.) Even so, the vast majority of those accesses are for the same small set of data. Things like file associations, etc. There would be no significant performance penalty if text files were used, as that data would be cached anyway (just like it is with the registry).
The registry also enables concurrent access and permissions control.
The registry is used for these things, yes, but there's literally nothing that makes it hard or impossible to do the same thing with text files, so that's not really an advantage of using the registry.
In my view, the registry is one of the top 10 worst things about Windows.