Read more: Cookbook data migration to version 4.0 für Joomla 2.5
I've found that Joomla! 2.5 might lead to system Time Out. I got this on a local Xampp system. e.g. when choosing Extensions -> Manage.
This was dependent of the number of extensions. It could though apply to oher funtions as data transfers. It could also be a problem on server systems.
Fix on a local Xampp system:
Open he file php.ini in path ... xampp/php/ Look for the line
max_execution_time = XX
I increased the standard value 30 (XX) to 90 and I didn't get the time out anymore.