SQL server in memory table performace
It is used only for selecting data. 
  
  ...
  
  The results I am getting in SQL
  Server profiler are rather dissatisfying.


That is because you did not read what issues memory-optimized tables do address.

They were not invented to "just keep your data in memory", if your table is relatively small and frequently used, it will be in memory after the first reading from disk without any "optimizing".

Memory-optimized tables are lock free, it means in highly concurrent environment instead of beeing blocking on modifying when someone reads the data, new versions of rows are inserted, so there is no blocking.

Your table is readonly, as you said, so what did you want to achieve? There is no blocking at all on this your table, so what did you want to resolve?

If you just did something without good understanding of what you are doing, I can suppose that your bucket count was also chosen wrong, and wrong bucket number slows down your SELECT.