Your problem is that lightroom is that its a complete hog. Even the iMac pro isnt smooth with it. Ive used 10+k systems that arent smooth with 26mp raw files let alone 50mp.
The secret is it all depends on the screen resolution.
Think about it a 5k display is 14mp and 60fps if your using anything like a 26+mp raw files and your moving each slider the UI will be slow because its trying to shift 14 million pixels 60 times per second, the lag comes because it cant keep up. The other problem is the graphics card is no where near fast enough for these sort of things.
The only way to make it work adequately is to use a lower resolution screen. A 2k display like a 27" ACD it should work fine. In all honesty at 2ft at 27" I dont think you would notice a huge difference but workflow speed will probably double.
Just try it, unplug the monitor and use the built in 15" display which is 2k and see what the performance is like. Same file same adjustments. Will be far faster. I wanted to get a new mac after switching to dell workstations 2 years ago, took a TB3 SSD with Mac OS into the apple store with lightroom installed and my catalogue. The 15 and 13" were fine on their built in displays but the 5k iMac which I could only test the low end and the mid range i5 was slower than my 2010 mac pro, the iMac pro was quicker but not base line 5k quick. Still very stuttery.
There is also a few other things you could do to optimize lightroom. Make sure you make 1:1 previews on import, click on lightroom catalog settings and change the discard 1:1 previews to never, try reducing the standard preview size and the preview quality to medium.
Also try lightroom it with graphics processing on and off.
Other things you can try: Because your not editing actual pixels but it is continually updating metadata to show you a preview, certain tasks are much more performance hungry. Sharpening, noise reduction, lens corrections etc. If like many, you have these set as an import preset then its instantly going to be slow. Same can be said for dehaze... that a real hog too.
For example my mac pro in my sig, with GPU turned off dehaze is super quick but everything else is laggy with the GPU turned on everything is much smoother and quicker but dehaze is awful. Thankfully I rarely use it because I dont like the look. My mac pro is a back up machine now because its just painful to use daily.
The other option is to use Lightroom CC not classic. CC is the mobile friendly version which doesnt give you as much editing possibilities but has been written from the ground up and is far faster and has a desktop version.
Unfortunate all of CC is a hog and Apple is not offering hardware to get the best out of it. Not necessarily apples fault but CC runs better on NVIDIA cards and high clock speeds. 3.5ghz and above. Its normal to see 500+ percent, just changing the exposure in lightroom will jump the CPU usage up to 600+ on my mac pro hex core machine.
The other issue are that the laptops use low powered CPUs and with no dedicated GPU the CPU works a lot harder.
An iMac wont be much better either that 5k monitor has a lot to answer for.
Its difficult because apple doesnt sell a modern headless machine you can add your own display too so your stuck with the iMac if you want a powerful desktop from apple and the graphics cards 560-580 are still only mid range cards not high end so its not like you will see a huge huge performance increase.
For your video editing it would be a better idea to get final cut, its optimised for the mac and far far faster. You can edit 4k footage on a low end macbook no problem.
My dual xeon dell workstations with quadros minces all of the apple line up and its 18 months old. The quadro is as much as a low end iMac 5k, that the nice thing about a PC is you can build a machine specifically for your uses. Creatives are the main buyers of macs and CC runs so badly on them because you get what apple thinks you need. Its not really apples fault its adobes poor optimization. With no pro options from apple over the last 5 years worth considering, more pros have moved to windows and with select hardware it runs far better. You do have to put up with windows tho which isnt as bad as it sounds. As CC is cloud based the UI is exactly the same, same experience on a different platform. The OS for work isnt as important as it used to be, getting work done quickly is.
