Performance issue with FBA pack

We have several thousands of users in our user memebership database. When we try to edit a user through the FBAPack (SharePoint Foundation 2013) it takes one minute to show the first screen with 30 users. When we search for a user it takes two minutes to find the user. Processor usage on the Sharepoint goes up to 30-60% during the user search query. A query on the SQL server takes less then 1 microsecond to search for a specific user. Do you have any idea why it takes so long in the FBA pack?
Yes, the issue is that the FBA Pack doesn't access the database directly, but goes through the membership provider. This ensures compatibility across multiple types of membership providers, not just the SQLMembershipProvider. Unfortunately the query options for the membership provider is pretty limiting, which requires us to retrieve all of the records from the membership provider. Not only that, but the membership users have to be matched up against SharePoint users once they're retrieved to access their user profile fields.

I've created an issue to investigate this further to see what can be done to improve the performance: