A hundred million keeps quiet, but of course the table is severely optimized for the particular quite simple query. InnoDB.
To switch to another DBMS, probably not worth it. It is worth to measure how dire the situation is now and whether to change it. Analyze what queries are coming to the table, how fast they work, what are the main operations are (insert-read), there is this funny effects (for example, locks on MyISAM and InnoDB are very different) if all the indexes are, are there any extra indexes, is it possible to reduce them, etc. then be able to make a decision. Now if You have 1 million, you can score a test database of 5 million and see if much has changed alignment.