Thanks for your reply. 
But my test result is:
1. OpenLDAP 2.4.38 on Windows is about 10x slower than it on Linux. Not 2x nor 3x as you say.
2. OpenLDAP 2.2.30(the previous version we used) on Windows is almost the same fast as Linux.
That makes me confused. Is there something related changed from 2.2.30 to 2.4.38? 
Or there something wrong i do with 2.4.38 on Windows?