test_lagrange in test_jit_forms.py runs very slowly (times out on Ubuntu focal 20.04)

[drew-parsons]

Running the ffcx tests, I notice that test_lagrange_triangle, test_lagrange_tetrahedron in test_jit_forms.py run significantly more slowly than other tests. This is particularly a problem on the PPA Ubuntu focal (20.04) build, where the build times out, breaking the PPA build of ffcx 0.3.0, cf. https://launchpad.net/~fenics-packages/+archive/ubuntu/fenics/+build/22073710
or https://launchpad.net/~fenics-packages/+archive/ubuntu/fenics/+build/22073710/+files/buildlog_ubuntu-focal-amd64.fenics-ffcx_1%3A0.3.0-1~fenics1~focal1_BUILDING.txt.gz
The PPA build for Ubuntu hirsute (20.10) completes successfully.

Are these tests expected to be slow, and the focal build should just skip them?
Or is it an indication of a bug either in ffcx itself or in the build configuration that ought to be fixed?
Or is it a constraint in the library versions available on Ubuntu focal (20.04) that users should be aware of (using this Issue to document the problem)?

[drew-parsons]

The problematic case seems to be [3-<lambda>-<lambda>0-double] (possibly other 3-<lambda> cases also).

It stalls the tests for test_lagrange_triangle as well as being the point where test_lagrange_tetrahedron is killed. Tests pass quickly up to test_lagrange_triangle[2-<lambda>-<lambda>2-double].