I mean what is important for a smooth use of CIE
lot of ram, lot of thread, a fast ssd or HDD or maybe CIE will use the GPU for those task.
because at 6k the CIE is not smooth. you can wait maybe 5 sc after scrolling a vallue or just check uncheck a light visibility
CIE uses CPU for its tasks. It is heavily multithreaded so you will get major speedup from CPU upgrade.
RAM capacity does not affect speed unless your system does not need to swap to disk, i.e. you have enough memory to hold whole uncompressed image including all its elements (plus memory to satisfy requirements of your other running programs and the system itself, obviously). For instance, 6k single-element EXR without alpha channel takes about 240 MB of RAM.
Using SSD affects only loading/saving. On my system loading from SSD is about 2x faster than from standard HDD.
Computing LightMix is not so much CPU intensive task but a LOT of data has to be processed. So speed of RAM is probably the bottleneck there. More light select elements you have, more data has to be processed. Maybe you can group your lights into smaller number of light selects?
On my system using CIE is pretty much smooth for 6k images (if not denoising at the same time). I have a Ryzen 7 1700. Is VFB also slow for you?