After Effects is designed to play back frames cached to RAM in real-time. some of the time it is capable of playing back in real-time without caching. this capability has been greatly improved in CC2015.3 And more in CC2017. the exception are certain mac systems but good news - there is a new update fix: https://blogs.adobe.com/creativecloud/after-effects-cc-november-2017-bug-fi... that is supposed to resolve the very annoying playback issues with 10.11 and 10.12 Os. see if it improves the situation on Yosemite as well.
Update CC 2017 to version 14.0.1 and then, once a preview is cached, it should play back real time again. The update fixed the problem that's been happening for some folks since CC 2015 first released.
- The Great Szalam
(The 'Great' stands for 'Not So Great, in fact, Extremely Humble')
No trees were harmed in the creation of this message, but several thousand electrons were mildly inconvenienced.