That could indicate a database related issue then. Just to be sure, check if the index table is indeed enabled.
I strongly suggest running a database repair and table optimize queries on the whole database to make sure the data was correctly imported and it’s not fragmented. 15 million rows is a lot, but not tragic. Fragmentation can cause a huge memory issue with the actual table index structure on the database level.
I would also suggest looking at the server memory and space usage, just to make sure it has enough resources.