I've noticed some slower calculations with PTC/SEHI/QBI as well.  But my lag is 5-10 seconds.  Still mildly annoying.  The closest thing to a "fix" that I can come up with is maybe have a worksheet like the Education Credit optimizer where the user has to click a box when they're ready to run the PTC/SEHI circular math calculation.  I don't like that solution either.  At least with the Ed credits you're still filing an accurate return even though it may not yield the optimal tax result.  With the PTC/SEHI you could end up with an inaccurate return if you just forget to go back and check the box.