diff --git a/ci/Dockerfile b/ci/Dockerfile
index 02b05f3..77e63db 100644
--- a/ci/Dockerfile
+++ b/ci/Dockerfile
@@ -71,7 +71,7 @@ RUN set -e; \
     && source ~/.bashrc \
     && mkdir -p "$(rbenv root)/plugins" \
     && git clone https://github.com/rbenv/ruby-build.git --depth=1 "$(rbenv root)/plugins/ruby-build" \
-    && rbenv install 3.4.0 \
+    && MAKE_OPTS=-j2 rbenv install 3.4.0 \
     && rbenv global 3.4.0 \
     && ruby --version \
     && which ruby \