[Icons] Docs: Adding info on enduser performance

Page: https://symfony.com/bundles/ux-icons/current/index.html#performance

Info is taken from #2782 (comment)

Author: ThomasLandauer

src/Icons/doc/index.rst

@@ -456,7 +456,12 @@ of the following attributes: ``aria-label``, ``aria-labelledby`` or ``title``.
 Performance
 -----------
 
-The UX Icons component is designed to be fast. The following are some of
+When including the same icon multiple times on one page, the same string (SVG content)
+is inserted repeatedly in the HTML source. However, since most icons are small (just a few
+hundred bytes), this overhead is negligible in most cases (especiall wthen proper
+transfer encoding like e.g. gzip is enabled).
+
+On the server side, the UX Icons component is designed to be fast. The following are some of
 the optimizations made to ensure the best performance possible.
 
 Caching