KDE Plasma 5.12 runs perfectly (buttery smooth, and VERY snappy) until multiple monitors are enabled. After enabling additional monitors the Kickstart menu takes several seconds to respond and then several more seconds to render properly. There's a similar issue when opening the shell menu with right-click. I've tried plasma 5.12 on several distros including the latest Kubuntu and openSUSE and the results are the same.

I'm unsure if this is a issue is particular to the current Plasma 5.12 release as I've been using Plasma 4 until now.

I'd appreciate any advice on how I can investigate this issue properly before I submit a bug report.

Okay, so Plasma 5 wasn't entirely the problem, although the issue did not exist with Plasma 4 or when booted to Windows 10 on the same system.

It turned out to be hardware/BIOS related. I fixed the unresponsive behaviour by changing primary display priority in bios, but the best result was from simply disabling iGPU in BIOS and then using my discrete AMD graphics card for all three monitors.