Hi Marc!
Since we received a message on the users list about this, I made some comparisons between 2.6.0-pl2 and 2.5.4. I am quite surprised to see 2.5.4 being 3-4 times quicker in routine operations like cookie login, changing db, going into Structure page for a table, etc.
Okay, I had that impression as well for quite some time. Nice to see someone verifying this.
I suppose the config import stuff and the DB layer, as well as some added regular expression magic may add up to the performance impacts.
I am working with xdebug on Serendipity (a blog tool) performance tuning, so I will try to make the same for phpMyAdmin as well :)
Regards, Garvin.