added a note about performances

extension/ui/pages/help.html

@@ -195,7 +195,8 @@
 					<li>
 						<span class="option">remove unused styles</span>
 						<p>Check this option to remove all CSS rules and styles that do not match any element. Checking this this option should
-							not alter the document and can considerably reduce the size of the file.</p>
+							not alter the document and can considerably reduce the size of the file. Checking this this option option may also
+							increase the CPU consumption and the time needed to process a page.</p>
 						<p class="notice">It is recommended to
 							<u>check</u> this option</p>
 					</li>