html - multiple php calls slowing page load? -

i rewriting html site php. there various changing menus want make php calls:

 <?php include("header.php");?> 

with footer, , sidebars have 4 or 5 php includes on each page.

how slowing pageload 5 php calls? if want fast load worth sacrificing sitewide editabitity , calling less php pages? or few milliseconds?

is there difference speed wise between calling 2 css files or 2 php files?

( caching system use such simple php calls? )

for static files, css files, merging them decrease page loading time. because these files not server-side files.

clients send more 1 request downloading these files. effect loading time. php files server-side files.

it won't effect loading time (if files not complicated much).