myisampack running really slow for large DBs
If you have any TEXT/BLOB fields in your MyISAM tables, you may just have bloated fields.

You verify this, go back to your older MyISAM tables and perform this:

SELECT LENGTH(TextOrBLOBField) FROM some_table;


Run this same query against more recent tables and see if the slowness is just O(n) (order n) running time based on the amount of recorded data.