some pages of my website have .php extension and other have .htm

Will this affect the downloading speed for visitors ?

No, but if you can work with PHP, do it. It will be much easier for templating

If your pages are just plain html there is no reason to work with PHP. It can’t provide anything for you but slow down your web site due to the extra resources it takes due to the php interpreter.

No there is no difference.

Yes, but the difference won’t be noticeable. By default, PHP parses all files with an .php extension before it sends the output to the browser. That means it checks for <?php tags and parses the content of them. (If any).

While .html files won’t be parsed by default. So they’re served directly.

You shouldn’t notice any difference in speed.

There is a big difference between php and html. It’s not the same type, so you can’t actualy ask .htm or .php.