Fulltext search takes more and more time as the number of documents is growing

This was rapported by some customers with around 50.000 documents in the DB.
After investigation, it happens since 5.5 due to changes from NXP-5689
Seems like the NX_TO_TSVECTOR function is called at each query whereas it should already be done in the index. Altering the FULLTEXT table columns to their previous type solves the problem because the code can handle both cases.