diff --git a/.hgarcanist/__phutil_library_init__.php b/.hgarcanist/__phutil_library_init__.php
--- a/.hgarcanist/__phutil_library_init__.php
+++ b/.hgarcanist/__phutil_library_init__.php
@@ -1,3 +1,15 @@
 <?php
 
 phutil_register_library('fb-hgext-arcanist', __FILE__);
+
+// When running under HHVM, reset xdebug.max_nesting_level to 0
+// This is unfortunately set to PHP_INT_MAX in
+// libphutil/scripts/__init_script__.php
+//
+// With HHVM, setting xdebug.max_nesting_level to a non-zero value
+// automatically turns on XDebug profiling.  This makes many things
+// substantially slower, and appears to even affect non-PHP children
+// processes spawned by this process.
+if (defined('HHVM_VERSION')) {
+  ini_set('xdebug.max_nesting_level', 0);
+}