Lightroom Classic and good performance was something you could not say in the same sentence until not too long ago. But things have started to change. 

Since I have done extensive testing and performance optimizations with Lightroom Classic for years, I thought I'd share a little blogpost to point out some of the biggest caveats and performance gainers from a hardware perspective for this particular piece of software.

General notes:

1. Lightroom Classic has gone through some serious code optimization from Adobe these last years. That means that you can now see some real performance gains from throwing better hardware at it. Especially good a GPU and highfrequency multicore CPU's now actually makes a difference. This was NOT the case with Lightroom 6 and even to some extent Lightroom 7 which was the first subscription only version.
   So if you are still using those versions, you can ignore this blogpost apart from this tip: There is nothing you can do apart from throwing a higher clocked CPU core at it. The old versions cannot use GPU acceleration, it will not use more than about 4 - 6 GB Memory and it does not use more than one - at best two - CPU core for almost all tasks. It does not scale with better hardware at all, and clock frequency is all that matters. Especially a 4K monitor kills LR6/7 performance, so consider staying at 2560x1600 or even better: 1920x1080.
2. Since v8.4 performance has increased substantially on the same hardware, and the current v9.3 is by far the best performing version of lightroom classic yet. So this thread is based on versions after 8.4

"So if you are still using those versions, you can ignore this blogpost apart from this tip: There is nothing you can do apart from throwing a higher clocked CPU core at it"

General observations:

1. Lightroom still has tasks that responds "only" to very fast single core performance, but especially since v9.2 several of those tasks received GPU offload/acceleration, and thus the need for a good GPU has increased - otherwise performance might suffer.
2. Lightroom now runs really well with a 4K main screen if you have a decent GPU, and I would highly recommend 4K for the added screen real estate - more image, less menus/toolbars in lightroom (see caveat about screen size).
3. Intel iGPU's is completely worthless in Lightroom (due to LR GPU acceleration features) and constitutes a huge performance bottleneck - especially at resolutions above 1920x1080.
4. Lightroom seems to respond about equally well to both nVidia and Radeon cards at this time (It used to be better with nVidia). It does NOT scale particular well with GPU "core" count, but rather with GPU frequency and memory. Recommended sweetspot is nVidia GTX1660 SUPER 6Gb or Radeon RX5700 8Gb (Preferably high clocked models). There seems to be very negligible/nonexistent performance gains above the RTX2070 or RX5700XT cards. There is quite a penalty for using lower end cards and 6 - 8Gb VRAM is highly recommended.
5. Lightroom does not see any tangible performance gains from placing images on SSD, so using a spinning HD for images is just fine. The Catalog (and included previews) responds very well to a SSD and is definitely a MUST.

"Intel iGPU's is completely worthless in Lightroom"

Caveats:

1. Lightroom performance on a dual monitor setup is HIGHLY performance sensitive. The second monitor is not GPU Compute accelerated, only UI redraw. So the resolution of the second monitor has a huge impact on general performance (especially if it's in Loupe mode - i recommend using it for GRID mode instead). I highly recommend staying at 1920x1080 or thereabout on the second monitor. Also, if you use a second monitor, make sure to have at least a nVidia 1660GTX 6Gb/Radeon RX 5700 8GB or better GPU.
2. Lightroom really needs memory - BUT: It does not scale with huge memory sizes. You will never see Lightroom use more than about 15 - 18Gb of memory on its own. So 16Gb is fine and anything above 32Gb is worthless unless you run several other applications at the same time.
3. Multicore CPU use above 4 cores is generally restricted to import/export and preview rendering. So CPU choice really comes down to whether you edit/develop a lot on relatively few images (fewer cores (4+) and higher frequency preferred), or if you have large quantities of images coming through Lightroom (more cores (8+) over highest possible frequency preferred).
4. Lightroom on 4K monitors does follow Windows/Mac screen scaling settings for its menus/toolbars. But: depending on your eyes, 4K only really starts to make sense @ 32" screen size or more. Otherwise you will have scaling settings in place that scales your toolbars/menus to a similar screen layout of a 27" 2560x1600 monitor - and thus loosing the biggest advantage of a 4K monitor.

Hope this can assist you in making better hardware choices for your Lightroom Computer :-)