doc: add in release note about compiler being roughly 15% slower

Updates #49569

Change-Id: Ifba769993c50bb547cb355f56934fb572ec17a1a
Reviewed-on: https://go-review.googlesource.com/c/go/+/370154
Reviewed-by: Austin Clements <austin@google.com>
Trust: Dan Scales <danscales@google.com>

doc/go1.18.html

@@ -321,8 +321,11 @@ <h2 id="compiler">Compiler</h2>
   new <code>go</code> command <code>-asan</code> option.
 </p>
 
-<p>
-  TODO: Mention build speed impact.
+<p> <!-- https://github.com/golang/go/issues/49569 -->
+  Because of changes in the compiler related to supporting generics, the
+  Go 1.18 compile speed can be roughly 15% slower than the Go 1.17 compile speed.
+  The execution time of the compiled code is not affected.  We
+  intend to improve the speed of the compiler in Go 1.19.
 </p>
 
 <h2 id="linker">Linker</h2>