Generally, you do not need to change much. You need to update your store URL in settings. Make sure you preserve permissions. copy all files and directories and clear cache after you copy.
Just tried on our demo and saving in the opening modal works fine. Note, if you have options for the product, you will not be able to edit quantities on the main level.
If you expect ext field to become editable on the click, it is not how it suppose to work.
I would strongly recommend upgrading to newer version of AbanteCart. Your version would indicate that you run on deprecated version of PHP and that is very insecure.
Generally, AbanteCart is well optimized to use backed resources and have fast/optimized SQL requests. Our team dedicated a lot of time for that. There are many other factors that can effect the performance that go beyond the code base. You need to look at hosting, versions of the PHP and database, network, etc.