This sounds like a bug that the Adobe team knows about and has promised a quick fix for. In the meantime, I'd suggest just using an older version until the next bug fix update (which is coming "soon"). Remember, you can install older versions alongside your current version.
Thanks. I'll re-install the older version.
CC 2014 (updated to version 13.2.x) has the most normal RAM preview behavior on Macs with the problems you described. Now, if you're running Sierra, you'll have to be aware of the line break issue and, possibly the Show Guides/Clear Guides problem. Not a big deal, just some changes made in Sierra that Adobe has addressed in CC 2017, but will be issues with older versions. (Again, there should be a fix for CC 2017 soon and then none of these problems will be a concern.)
Everybody who is having this issue, please install the latest update that came out last night: After Effects CC (November 2017) Bug-Fix Update Is Now Available
This update is 14.0.1 and should be a bug fix for this and several other disturbing issues - especially for Mac users. It does not fix all the bugs that are new with the CC 2017 release (Known issues in After Effects CC 2017), but it does fix some issues that have been plaguing After Effects since the first parts of the new architecture showed up in the CC 2015 release.
If you can't see the update yet, you may need to wait a while until you see it as it can take up to 24 hours to populate an update across all the data centers. You may need to log out, relaunch the Creative Cloud app, and log in again.