DW CC 2015: unmodified files take many seconds to close; DW unresponsive

DW CC 2015, MacOS 10.12.1 Sierra

Observations on two computers running DW CC 2015: My primary machine-- I recently installed DW CC 2017 on it, then reinstalled CC 2015 in the past 3 days.  (Due to a show-stopper on CC 2017, I guess I'm going to be needing to use CC 2015 for the time being.) My secondary machine-- I use  it only for a few hours a week. It has has been running CC 2015 since that was released.  I haven't installed CC 2017 on this machine.  

The Experiment:  Open, say, six existing html files in DW.  Make no modifications. Then use the 'x' button in their tabs to close them, one by one.  Verify:  the 'x' button inverts its white and black to confirm the button IS activated.  The tabs should close immediately -- that's what I'm 99% certain I've observed in the past, for years, through many DW versions. 

In the last two days, on my primary machine:

• Occasionally files close immediately, but there are  frequent  delays of 6-7 seconds and longest delays of over 15 seconds.  DW is unresponsive during the delay.  UNACCEPTABLE!  

•  Console messages don't offer any help.  I checked CC 2015 CPU data in Activity Monitor.  It show DW CC 2015 idling at about 2% of a CPU most of the time --seems reasonable.   In cases some case, I see a brief jump to about 50%, sometimes as high as 100%+, which seems a lot of CPU for closing an unmodified file, but perhaps still reasonable.  In general, yes, significantly more CPU used when tab closing is delayed, but I can't say much more, no further correlation.

• Cache problem?  I applied the fix here  and this  seemed to reduce the wait times to a maximum of 6-7 seconds --for roughly overnight.  Now, seldom, tabs close right away.  But the long delays returned.

• Exiting and re-launching DW has no effect.  Restarting MacOS has no effect.

On my secondary machine, checked just now:  

• I observe frequent closing delays in some cases of 6 or so seconds.   In some cases, the tabs disappear immediately.   I'm fairly certain I would have noticed a frequent 6+ second freeze in DW.   

Again on my primary machine: 

• I ran the same experiment multiple times on CC 2017.  In every case, tabs of unmodified files closed immediately.   Repeat:  In every case, tabs of unmodified files closed immediately.

I installed 10.12.1 Sierra on both machines shortly after it was released, so I suppose the problem might be due to an incompatibility between it and DW CC 2015.  That wasn't too long ago, but long enough that I'm pretty sure I would have noticed this issue on one or both of my machines.

I think something changed in the last 3 days, and it seems very unlikely this occurred on both my machines.  (So: Standard fixes like deleting all DW CC 2015 preferences seem very unlikely to be effective.)

Ideas?  TIA