diff --git a/projects/llvm_libcxx/build.sh b/projects/llvm_libcxx/build.sh
index 8121900ac..13c28cc4c 100755
--- a/projects/llvm_libcxx/build.sh
+++ b/projects/llvm_libcxx/build.sh
@@ -18,6 +18,8 @@
 if [[ $SANITIZER = *undefined* ]]; then
   CXXFLAGS="$CXXFLAGS -fsanitize=unsigned-integer-overflow -fsanitize-trap=unsigned-integer-overflow"
 fi
+# trace-cmp makes this target too slow.
+CXXFLAGS="$CXXFLAGS -fno-sanitize-coverage=trace-cmp"
 
 for f in $(grep -v "#" libcxx/fuzzing/RoutineNames.txt); do
   cat > ${f}_fuzzer.cc <<EOF