diff --git a/.github/workflows/tests.yml b/.github/workflows/tests.yml
index 13746608d4..12420911b4 100644
--- a/.github/workflows/tests.yml
+++ b/.github/workflows/tests.yml
@@ -638,9 +638,10 @@ jobs:
       - name: Prepare Complement's Prerequisites
         run: synapse/.ci/scripts/setup_complement_prerequisites.sh
 
+        # use p=1 concurrency as GHA boxes are underpowered and don't like running tons of synapses at once.
       - run: |
           set -o pipefail
-          COMPLEMENT_DIR=`pwd`/complement synapse/scripts-dev/complement.sh -json 2>&1 | synapse/.ci/scripts/gotestfmt
+          COMPLEMENT_DIR=`pwd`/complement synapse/scripts-dev/complement.sh -p 1 -json 2>&1 | synapse/.ci/scripts/gotestfmt
         shell: bash
         env:
           POSTGRES: ${{ (matrix.database == 'Postgres') && 1 || '' }}
diff --git a/changelog.d/16520.misc b/changelog.d/16520.misc
new file mode 100644
index 0000000000..ea10fd4345
--- /dev/null
+++ b/changelog.d/16520.misc
@@ -0,0 +1 @@
+Enable dirty runs on Complement CI, which is significantly faster.
diff --git a/docker/complement/conf/start_for_complement.sh b/docker/complement/conf/start_for_complement.sh
index 5560ab8b95..7b012ce8ab 100755
--- a/docker/complement/conf/start_for_complement.sh
+++ b/docker/complement/conf/start_for_complement.sh
@@ -68,6 +68,11 @@ if [[ -n "$SYNAPSE_COMPLEMENT_USE_WORKERS" ]]; then
 
   fi
   log "Workers requested: $SYNAPSE_WORKER_TYPES"
+  # adjust connection pool limits on worker mode as otherwise running lots of worker synapses
+  # can make docker unhappy (in GHA)
+  export POSTGRES_CP_MIN=1
+  export POSTGRES_CP_MAX=3
+  echo "using reduced connection pool limits for worker mode"
   # Improve startup times by using a launcher based on fork()
   export SYNAPSE_USE_EXPERIMENTAL_FORKING_LAUNCHER=1
 else
diff --git a/docker/conf/homeserver.yaml b/docker/conf/homeserver.yaml
index c46b955d63..c412ba2e87 100644
--- a/docker/conf/homeserver.yaml
+++ b/docker/conf/homeserver.yaml
@@ -67,8 +67,8 @@ database:
     host: "{{ POSTGRES_HOST or "db" }}"
     port: "{{ POSTGRES_PORT or "5432" }}"
 {% endif %}
-    cp_min: 5
-    cp_max: 10
+    cp_min: {{ POSTGRES_CP_MIN or 5 }}
+    cp_max: {{ POSTGRES_CP_MAX or 10 }}
 {% else %}
 database:
   name: "sqlite3"
diff --git a/scripts-dev/complement.sh b/scripts-dev/complement.sh
index 3e0cddb527..b1a8724b7e 100755
--- a/scripts-dev/complement.sh
+++ b/scripts-dev/complement.sh
@@ -216,6 +216,10 @@ extra_test_args=()
 
 test_packages="./tests/csapi ./tests ./tests/msc3874 ./tests/msc3890 ./tests/msc3391 ./tests/msc3930 ./tests/msc3902"
 
+# Enable dirty runs, so tests will reuse the same container where possible.
+# This significantly speeds up tests, but increases the possibility of test pollution.
+export COMPLEMENT_ENABLE_DIRTY_RUNS=1
+
 # All environment variables starting with PASS_ will be shared.
 # (The prefix is stripped off before reaching the container.)
 export COMPLEMENT_SHARE_ENV_PREFIX=PASS_
@@ -274,7 +278,7 @@ fi
 export PASS_SYNAPSE_LOG_TESTING=1
 
 # Run the tests!
-echo "Images built; running complement"
+echo "Images built; running complement with ${extra_test_args[@]} $@ $test_packages"
 cd "$COMPLEMENT_DIR"
 
 go test -v -tags "synapse_blacklist" -count=1 "${extra_test_args[@]}" "$@" $test_packages