Hello Paul,
The settings page is getting slow because the PHP post limit may be about to exceed, this is a server limitation.
To counter this,use the CSV feature fro adding or editing rules.
The ideal way to use this is, first frame 4-5 rules in shipping pro.
Test them whether they are working. Once you are convinced that they are working, click on Export CSV. Take that CSV and add in more rules using MS Excel or any other tool. After that,import the CSV.
This will allow you do your shipping properly. For more queries raise a ticket at support.xadapter.com