Blocking all IP's except US & Canada

I am looking to make changes to my .htaccess file so that all ip addresses are set to deny and I will explicitly allow only ip addresses from the US & Canada. I am able to discern which IP addresses are allocated to these two countries from the following URL:

http://www.ipmaster.org/ranges.php

I have two questions however.

1) How often are ip addresses reallocated, moved are added to a specific countries block? For instance, is there ever a conceiveable scenario where the US would take over an IP block that was previously allocated to Mexico? Will specific US IP ranges ever fall out of service or move to new countries? Are all IP addresses accounted for? Will the US be adding new IP's moving forward?

2) This will require a good number of entries in my .htaccess file. Will this in effect slow down the web site for all US users to the point where I will loose any benefit I will have received from keeping foreigners out?