Help, urgen! db store file very slowly after add 80000 files.

hi,all , anyone have some experience on continueously add files to db with restxq web service interface.? I have a restxq service to add post xml data to db, in db/path1/path2/{pid}.xml. at begining, its very fast, after to 90000, slowly ,bout 2 files per second . how to increase the speed? autoindex is off.