Currently CSF is using separate calls to support ipset and with a big list of blocked ip-adresses (25000) this takes a long time to add al of them to ipset.
Before the CSF ipset support we where using our own implementation of ipset and we are using the restore method of ipset to import the list very fast. What you do is create a file with al of the add calls like this.
Thank you for the suggestion. We will look into implementing this in the next major release - we do already use ipset restore for some functionality but not part of FASTSTART.