It would be more efficient yes, however the SQL implementation supports more than just MySQL, and so it’s not easily doable.
I administrate a number of large networks running LuckPerms - we have close to 1 million entries in one of our user permission tables (we store a lot of data using permissions 
), and the disk usage for the table + indexes is still negligible.
I’m wondering if something else is causing your issue? Could you perhaps share some stats?