Q: New imac5K i7 preview app super slow raw photos? Lightroom5 super slow? very disappointed

Neither am I flying a helicopter. I have no idea why you think that's relevant. I'm just trying to explain to you why it's incorrect to tell the OP that they are running out-of-date software when they are not. And frankly, if you understand that Raw images a re different what did you hope to demonstrate by downloading unspecified Canon and Hasselblad raws?

Speaking of meaningless, telling users that software runs on a machine you don't own, on an operating system you don't run, is a perfect example.

My friend, you're making assumptions based on a profile that has not been updated in several years. Frankly, I'm just not the anal about it. But then, you've been making inaccurate assumptions all along. So, why change now?

What I don't see in your reply is "Yes, I own a Late 2015 5k Retina iMac that I run OSX 10.11.2 on"

So far, you posted a link to show Lightroom 5 runs on El Capitan when the only three users commenting says it doesn't

You also ignored the link I posted from Adobe which lists various problems running Lightroom on OSX 10.11.2

Way to deflect the subject. At this point even if you said you did I wouldn't believe you.

Have a nice day.

Such as sensitive puppy you are.

Must admit I also didn't know Lightroom 5 was incompatible with El Capitan. I'm still using Lightroom 4 with El Capitan on my late 2015 5K iMac. Seems to work normally.

But what do you know? You're just using it...

In the case of Lightroom, the answer may be obvious, but do you have "Use Graphics Processor"

checked in Lightroom preferences under Performance?  If you do, uncheck it and see if that

makes any difference in performance (either better or worse).

If it doesn't make any difference or is actually better, then maybe there is some graphics driver/interface/OS

issue.

As to Preview having an issue, it could also be a similar situation, although the OS X RAW renderer will be

quite different than Lightroom.

FWIW, although I do not have your machine (late 2013 27" 3.2GHz i5 iMac, 32 GB RAM),

my 24 megapixel Nikon Raws open almost instantly.  Also, in Lightroom CC 2015.3 (Camera RAW 9.3),

it takes a second or two to open an image first time in a session but then pretty much instantly after that

(maybe something to do with LR caching).

Also, when you say all 8 cores are going full out, what processes are using up the all that CPU?  Perhaps the

GPU isn't being leveraged to accelerate the rendering?

Another quick question, do you have a standard hard drive, Fusion drive, or SSD?

Then again, as previously mentioned, Lightroom 5 is pretty old in terms off software these days.

Out of curiosity: I do not believe you mentioned which camera(s) you use? This may not matter since you are using third party apps, but, just in case, here is a link to the list of El Capitan's RAW compatibility with camera models:

Digital camera RAW formats supported by OS X El Capitan - Apple Support