Hi charloub, when it comes to regenerating 2700 images, keeping them to a certain size and using a proper naming convention will help the regenerating process.
There could be other limiting factors behind when it comes to such a large number of images. For example the database size can influence the process, the php version, memory allocated to WordPress by the host and the server hardware configuration etc.
What we are doing now is carrying out a process of elimination.
Lets hope this helps you and yes it is going to be a tedious task for sure however it is the only way to test this issue.