Steve Jenks from Indesign Secrets posted an answer to this and a link to another forum question both of which can be found here:
However to save you time I've copied and pasted his reply:
This has been discussed recently on the Adobe forums here
But the gist of it is – if the panels that are slow to open (they open eventually but take quite a while) then there is something else that you may like to try.
Open Activity Monitor and quite the ‘fontd’ process. This is something that has been causing problems for Indesign CS6 for some years now.
So quit Indesign, open Activity Monitor and quit ‘fontd’, then relaunch Indesign. It should be back to full speed.
You could also look at whether you might have duplicate fonts in your system. Since Mavericks this has also been the cause of this sort of behaviour in Indesign.
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The most important thing here is that CS6 is completely unsupported under Sierra (and El Capitan). Regardless of any troubleshooting you do, you are still likely to have issues. In short, you are on your own and the best advice is still to go back to 10.10 or earlier.