Second round of review is taking longer than the first one
Lots of things can happen to delay the review process. For instance, heavy teaching commitments in the winter can slow things down, although they should now be over. Also, there really should be no correlation between the reviewing times for an original submission and a resubmit after major revision. The referee has to essentially start from scratch.

But after three months, I would say you are entitled to a polite reminder letter to the editor.