More importantly, I wonder how slowing down EQ by a factor of 2 can
end up costing 10% of runtime when running the test suite. I think
this deserves investigation.
Do you have an idea how this could be investigated?
Usually such slowdown don't come from everywhere at the same time. So
you need to "slice" our total time into subelements, and presumably some
of those elements show a higher slowdown, so you can focus on those and
start slicing them further.
As you get closer to the source, the slowdown should become more marked.