Not sure how much is known or published about the back end of CircuitLab, but:
While simulating a different circuit, I noticed very long simulation time. I narrowed the culprit down to the below portion. With a DC sweep on V1 from 0V to 5V and a relatively large step of 0.1V it still takes several minutes:
Changing the Op Amp or BJT model didn't make a difference.
Simply replacing the BJT with a MOSFET speeds up the same simulation so that it runs in a fraction of a second.
Any ideas what's tripping up CircuitLab? Is it just that the BJT model implementation is more complex than the MOSFET?
(Please no answers telling me to use different software.)