OK, thanks...

so it's confirm :

1/ try to avoid table with big number of fields (mostly try to make the size of a record (size used by all fields) more little as possible)

2/ try to use a pool of parametized statement (read only read commited)... because here it's only the prepare that is slow

3/ try to use bigger page size (16k?)

but just want to know if in the background, in the engine, nothing can be done in the c++ code to ameliorate this behavior of the prepare