diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 121e9f75f..ab9c76f57 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -41,7 +41,7 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci-${{ github.job }}
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: .github/workflows/install_deps.sh amici
@@ -60,7 +60,7 @@ jobs:
         file: ./coverage.xml
 
   mac:
-    runs-on: macos-12
+    runs-on: macos-latest
     strategy:
       matrix:
         python-version: ['3.12']
@@ -80,14 +80,14 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: .github/workflows/install_deps.sh amici
 
     - name: Run tests
       timeout-minutes: 30
-      run: tox -e base
+      run: ulimit -n 65536 65536 && tox -e base
 
     - name: Coverage
       uses: codecov/codecov-action@v3
@@ -116,7 +116,7 @@ jobs:
         path: |
           ~\AppData\Local\pip\Cache
           .tox
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: |
@@ -149,7 +149,7 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci-${{ github.job }}
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: .github/workflows/install_deps.sh amici pysb
@@ -193,7 +193,7 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci-${{ github.job }}
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install julia
       uses: julia-actions/setup-julia@v1
@@ -242,7 +242,7 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci-${{ github.job }}
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: .github/workflows/install_deps.sh ipopt
@@ -278,7 +278,7 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci-${{ github.job }}
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: .github/workflows/install_deps.sh amici
@@ -314,7 +314,7 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci-${{ github.job }}
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: .github/workflows/install_deps.sh amici
@@ -350,7 +350,7 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci-${{ github.job }}
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: pip install tox pre-commit
@@ -382,7 +382,7 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci-${{ github.job }}
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: .github/workflows/install_deps.sh doc amici
@@ -415,7 +415,7 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci-${{ github.job }}
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: .github/workflows/install_deps.sh amici ipopt
@@ -445,7 +445,7 @@ jobs:
         path: |
           ~/.cache/pip
           .tox/
-        key: ${{ runner.os }}-${{ matrix.python-version }}-ci-${{ github.job }}
+        key: "${{ runner.os }}-${{ runner.arch }}-py${{ matrix.python-version }}-ci-${{ github.job }}"
 
     - name: Install dependencies
       run: .github/workflows/install_deps.sh amici
diff --git a/test/sample/test_sample.py b/test/sample/test_sample.py
index a33adde18..4f1d6fb86 100644
--- a/test/sample/test_sample.py
+++ b/test/sample/test_sample.py
@@ -3,7 +3,6 @@
 import os
 
 import numpy as np
-import petab
 import pytest
 import scipy.optimize as so
 from scipy.integrate import quad
@@ -11,10 +10,8 @@
 
 import pypesto
 import pypesto.optimize as optimize
-import pypesto.petab
 import pypesto.sample as sample
 from pypesto.C import OBJECTIVE_NEGLOGLIKE, OBJECTIVE_NEGLOGPOST
-from pypesto.sample.pymc import PymcSampler
 
 
 def gaussian_llh(x):
@@ -96,6 +93,10 @@ def rosenbrock_problem():
 
 
 def create_petab_problem():
+    import petab
+
+    import pypesto.petab
+
     current_path = os.path.dirname(os.path.realpath(__file__))
     dir_path = os.path.abspath(
         os.path.join(current_path, "..", "..", "doc", "example")
@@ -187,6 +188,8 @@ def sampler(request):
             n_chains=5,
         )
     elif request.param == "Pymc":
+        from pypesto.sample.pymc import PymcSampler
+
         return PymcSampler(tune=5, progressbar=False)
     elif request.param == "Emcee":
         return sample.EmceeSampler(nwalkers=10)