purge history stats taking long time and very slow

Hi All,

I'm trying purge history stats before doing upgrade of my 11.2.0.3 to 12c database, and it is taking a long time (more than 5 hours) to complete, any idea how to speed this up?

exec dbms_stats.alter_stats_history_retention(0);

set time on;

set echo on;

exec DBMS_STATS.PURGE_STATS(systimestamp);

Thanks!