slow queries against dba_data_files after dropping some datafiles

 Our production database has many, many datafiles, over 40,000.  Queries such as "select file_name from dba_data_files where tablespace_name = 'DATA_1;" formally executed in 30 seconds to 1 minute.  We have sinced dropped about 1170 datafiles, and now such queries take almost 15 minutes.  Since we had also added memory/CPU to the database server, I gathered system statistics, but this made no difference to the execution time of this query.  This is of very much concern, because we are fairly soon going to be dropping another 2500 datafiles.  Do I need to gather fixed object statistics?  If so, when should it