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System slowness during coalesce cleanup

Hi All ,

I am here to get solution for one of performance degrade issue .

Oracle Version : Oracle Database 12c Enterprise Edition Release - 64bit Production

RAC database having 2 Node cluster active-active system .

We have a partitioned table which will hit first from the application to the database .Its a partition table and we are supposed to keep 

only last 3 days data .On sunday we have dropped partition for 5 days and next day morning around 3:15 system slowness reported and 

when we checked the table which we have dropped partition is having some indexes and sys job is running below alter statement to Global Index Maintenance

to clean up orphan entries .

alter index Index_name coalesce cleanup;

The same slowness happened on tuesday morning at same time but i wondered we have not dropped any partition on Sunday.I have looked into AWR report but i got only that insert into the table took more time .

Could you please help me to understand 

will coalesce clean up will degrade system .

Why its caused even when we have not dropped partition .

Thanks for all your answers .