One of the key performance improvements that can be implemented for AX4 (and should apply to AX2009 also) is to disable the SysDeletedObjects40 (label is "keep updated objects" in en-us) configuration key.
In older version of AX, memo fields were used on a lot of the main tables (memo field can cause performance issues), in AX4 these were marked as obsolete but not actually removed, obviously if a customer was using these fields them they couldn't just disappear. So to remove the field it is necessary to disable SysDeletedObjects40 and synchronise the database.
Note that you'll still see DEL_ objects in the AOT but they won't exist in the database. Also note that one of the CRM tables is DEL prefixed but isn't assigned to this configuration key so won't be removed.