The member numbers were renumbered during the import. Any new ones will just auto-increment. For example if you now delete member #23. That member number will no longer exist and won't be used in the future.
As for the slowness. More than likely it's the MySQL server. If you turn on debug information in the control panel, down at the bottom you should get some information like you see here:
Generated in 0.081 seconds in which 0.005 seconds were spent on a total of 12 queries. Zlib compression enabled.
That will tell you where the slowdown is coming from, either the processing time of the scripts or from MySQL.
If it's MySQL, then there are a variety of questions to come after.
