why did FULLTEXT initialization took so long?

[lisheng1 zhang1]

We are using mySQL 5.7 Fulltext search and find it took a long time for simple query like:

select SQL_NO_CACHE ID from OBJECT_SEARCH where match(DATA) against ('+bvgs' in Boolean mode) limit 10;

in profiling it shows:
Status | Duration | CPU_user | CPU_system | Context_voluntary | Context_involuntary | Block_ops_in | Block_ops_out | Messages_sent | Messages_received | Page_faults_major | Page_faults_minor | Swaps | Source_function | Source_file | Source_line |
| FULLTEXT initialization | 17.650931 | 3.108000 | 0.724000 | 14983 | 1980 | 460608 | 9888 | 0 | 0 | 10865 | 60683 | 0 | init_ftfuncs | sql_base.cc | 9867 |

In manual it said FULLTEXT initialization is for (The server is preparing to perform a natural-language full-text search)

But we have never performed natural-language FTS (we only do Boolean search)? why we need this step?

Thanks very much for your guidance