Just a heads-up: at a client I ran into a situation where missing index tags caused extreme slowness.

In my case a table (in a DBC) with just under a million rows has a CDX file which was supposed to have 8 tags: one on the primary key plus 7 others. Somehow, the 7 non-PK index tags were no longer present, only the PK tag remained.

With the "bad" CDX a test report took 22 minutes to run.

When replaced with a "good" one and reindexed, it took 5 seconds to run (250x faster).

Over the years I've encountered a number of corrupted CDX scenarios. However they've always been such that the app won't start at all or crashes quickly with an obvious index problem. This is the first time I've run into tags that just silently went missing, without causing any functionality problems with the app (other than slowness).