You've found a bug. We can reproduce this problem which is just as you describe.
We've no idea why it's failing and will need to look into it deeper.
But I'm curious as to preview performance from your non-SSD drive. Using Rapid Preview I would expect realtime playback of 1080/2K footage even from a spinning disk. Certainly an SSD would give a lot more margin, but it shouldn't be necessary. I just tried an older MacBook with 5400 RPM drive, and could preview 1080 footage in realtime using Rapid Preview.
But you're not seeing that, right?
Hi Bob, my apologies, my original statement was ambiguous. It's not the playback speed in Rapid Preview that is very slow, but the preparation time.
Regular playback in Color Finesse was too slow to be useable for me (it took 54 seconds to play a 12.2 second clip, which works out to about 5.5 fps), whereas AE plays back at an average speed of 11 fps before it has loaded the clip into ram.
Because of this I decided to try the rapid preview instead, but loading up the rapid preview took even longer than playing the whole clip in regular preview. There was 65 seconds of prep time to play the same 12.2 second clip in real-time. I was hoping that moving the rapid preview cache to my external SSD would help speed things up, as my internal drive is a fusion drive (7200rpm drive combined with an SSD), rather than a pure SSD.
As an aside, is there a reason that previews are so much slower in Color Finesse than in After Effects? I don't have a professional computer by any means, but I do have a fully specced out 2014 iMac (4 GHz i7 processor, 32 GB RAM, and AMD M295X 4GB GDDR5 graphics card) and was kind of hoping I'd get better playback speed in Color Finesse than just 5 fps, seeing as the speed in AE is twice as fast.
OK, I understand now. Changing the cache location probably won't speed up the Rapid Preview rendering phase, but could help in the playback phase, so it should definitely be selectable (i.e., we need to fix that bug).
As to the rendering phase speed, Color Finesse has to request frames from After Effects so it will never be faster than AE. There will also be some additional overhead as the frame is transferred from AE, which will slow things down a bit.
That said, we've had some reports that the 3.0.14 update to Color Finesse slowed down the rendering speed somewhat (compared to earlier versions of Color Finesse). This is likely because of a fix we made to accommodate the more aggressive power management in OS X 10.10 and later, and Win 8.1 and later. We're currently looking into how we can tweak performance and still not run afoul of the latest OSes. I don't have a timeframe for this as we're still investigating. But it's on our radar!