Remove claim about 10x speedup

That kind of speedup is only achievable if also the user headers are
excluded from analysis. A follow-up patch could make clang-tidy
ignore declarations on files not matched by the HeaderFilter option.

Author: Carlos Gálvez

clang-tools-extra/docs/ReleaseNotes.rst

@@ -89,8 +89,8 @@ Improvements to clang-tidy
 --------------------------
 
 - It no longer processes declarations from system headers by default, greatly
-  improving performance (up to 10x speed-up). This behavior is disabled if the
-  `SystemHeaders` option is enabled.
+  improving performance. This behavior is disabled if the `SystemHeaders` option
+  is enabled.
 
 New checks
 ^^^^^^^^^^