PHP - page loading times longer than html?

Sorry if this is off topic but I'm renewing my site and would like to switch
over to PHP to be able to use server side includes. These seem to be less
time consuming than using templates and library items. However I wonder
about page loading times. I uploaded a test page and I've noticed that
although the page appears to have loaded there are messages in the status
bar when viewing the page with Firefox. Rather than seeing "Done" I see
"Read..." or "Waiting For...". This doesn't seem to happen with Internet
Explorer 6.0. What is happening here. Is something preventing Firefox from
loading the page correctly or is it because the server side include isn't
working correctly or something else? If I use server side includes will this
slow the page loading time down compared with using an htm page and a
library item? Very new to this.

Thanks,

Chris (Hunt)