It's tricky trying to help when the your problems are stated in such a vague way (eg. "Using LR6 and PS was like dragging a sticky thing through molasses" and "LR is dog slow. Every change I make manifests itself in blocky rendering"), altho' I appreciate it's not easy to verbalise computer symptoms.
What is your level of experience/confidence with computers?
Is the problem solely with LR, or are you having problems using other apps?
Is LR slow when working with files on the iMac's internal hard drive (you've stated that all your photos are on an external drive.)
Can you provide a screenshot (Shift Cmd 3) to show us this 'blocky rendering'?
What have you done so far? We know that you've exchanged computers, flushed the PRAM and turned off LR's GPU use.
What you're experiencing isn't normal and it's tricky to offer constructive help with so little information.
Hopefully the helpful folks here will help find a solution.
Just editing a basic picture mine doesn't drop below 85%. There must be something that's dragging the performance down?
Apple must be able to advise of basic checks. Can they not offer a remote service and check it out themselves?
Where did you purchase from Nick? What's strange of course is that this is your replacement machine, right? I think they have a duty to direct you to a local repair centre for someone to look over. If they can't diagnose remotely then that must be the next logical step.
The machine accesses the hard drive which is 2TB but only contains 256gb of photos. I have done a larger screenshot of the memory tab
Edit, yes I'm using an existing catalogue on LR. Could that be the cause of the problem I wonder?
I would suggest it's something to do with your external hard drive rather than the i.mac ,you said a few posts back you disconnected the drive and it could no longer read the files , the way I read it then is your working from stuff stored on the external drive rather than on the actual i.mac .
And as it's happened to two machines now I would think it's where the problem lies either external hard drive ,the connecting cables ,or configuration . What puzzles me is why are you not working from the i.macs hard drive
I actually think you're right. I didn't know/understand where Lightroom was pulling the photos from. It just never occurred to me. Since I moved the catalogue to the Mac HD and created a new one it seems much faster. Though ironically, Silver Efex is worse!
Just a thought … the disk you have your photo’s on did you take it from a Windows PC.
If you did it could be that that disk is configured for Windows. … the following article explains it better than I can. http://www.macworld.com/article/300...mac-drive-from-a-windows-or-linux-system.html
Or the external hard drive could be failing -- backups?
It could be. It's only 6 weeks old though with barely any files on it
Did you manage to get to the bottom of the issue?
OT. I'm running Mountain Lion in a Parallels virtual machine on Sierra (for FileMaker Pro 12 which is not supported in Sierra) and I have to say everything seems as fast as in the Sierra host. This in an oldish Mac mini with 16gb ram.>< snip><
TBH I wish I'd never upgraded from Mountain Lion. With every new OS I seem to get more glitches and more spinning wheels. I hate to jump on the bandwagon but I can't help feel that Apple really has gone downhill since Steve Jobs passed away. The main reason for me buying apple products was how smooth they are/were, but for me they're getting more glitchy. Even the iPhone now has glitches (tried 3 iphone 7's)