The idea behind FROID is that it removes some restrictions around scalar valued functions.
1. They can be inlined into the query, not run per-row returned
2. They don’t force serial execution, so you can get a parallel plan
If your functions already run pretty quickly over a small number of rows, and the calling query doesn’t qualify for parallelism, you may not see a remarkable speedup.
Even in that case, Erik argues that you can still get some benefits from SQL Server 2019 bringing those scalar UDFs inline.