Migrating to 4.5.1 or any newer release is extremely slow

I've made a series of migrations in order to see why the DW is so slow in the latest versions.

These are my results (from - to: duration):

• 4.2 - 4.3: 6min on Jetty
• 4.3 - 4.4: 4min on Jetty
• 4.4 - 4.5: 4min on Jetty, 3min on Tomcat
• 4.5 - 4.5.1: 5min and counting being stuck at "Applying UPGRADE for extension [Administration Application 4.5.1] on namespace [Main wiki]" on both Tomcat and Jetty
• 5.0.1 - 5.0.2: I waited for 1h25min and the migration still wasn't finished

Sorin also tested migrating from 5.0.2 to 5.0.3 on his home PC (Windows 7 32-bit, Tomcat 7.0.40, Intel Core i7-2700K @3.50Ghz) and it took him 60min to complete the migration.

On Ubuntu it seems to be faster than on Windows, but it still looks slow comparing to a 3 minute migration:

• 5.0.1 - 5.0.2: 17min on Tomcat & Java 6
• 5.0.1 - 5.0.2: 18min on Tomcat & Java 7

When I did the 4.2 - 4.3, 4.3 - 4.4 and 4.4 - 4.5 migrations I had many merge conflicts, but on the slow migrations there were none. My guess is something changed between 4.5 and 4.5.1 in the DW that had a major performance impact.