The reason PHP files are called whatever.php is so that they are run by the server instead of just being displayed.
The difference in time, isn't great. In fact, probably not noticeable at all, but it is better to have pages which do not include PHP code, named as whatever.html
It's just kinder on the servers.
As for old links, if you were feeling particularly clever, you could write a .htaccess file and put it in your public_html directory.
Something like this would go in it:
ErrorDocument 404 /errorpage.php?error=404
Then, make errorpage.php and use "HTTP_REFERER" to get the name of the page that the user came from (this would hopefully display the page they were trying to get to).
Then, put HTTP_REFERER into a string, cut off the .html and rename it to .php
Then redirect the user to that page.
If you're good at php, that should work