The EBX purge can be slow since EBX 6.2.0 because it can generate a SQL query which has a significant cost on a growing G_BLK table.
This occurs because the 'archive_id' column of this table is not indexed.

A change request was created to optimize the purge performance in EBX 6.2.3 : 

• CP-28490 : The 'archive_id' column of the G_BLK database table is not indexed.

For versions lower than 6.2.3, the workaround consists in creating a temporary index on the G_BLK table.
For example :

       CREATE INDEX tmp_idx_ebxblk_archive ON EBX_G_BLK (archive_id);

This temporary index must be removed prior to upgrading to EBX 6.2.3 or upper version.

EBX purge can be slow since version 6.2.0 because of a missing index on the database.