According to http://wolthera.info/?p=802 and https://docs.krita.org/Soft_Proofing, soft proofing and out-of-gamut warnings are now implemented in krita.
It seems those features will be present in krita 3.1.

I guess I could avoid colors outside the sRGB color gamut on 1.0-gamma ACEScg/rec.2020 canvases if I drew while soft proofing and out-of-gamut warnings were turned on.
If I drew while out-of-gamut warnings were turned on, would I see inaccurate colors or misleading warnings?
Will out-of-gamut warnings slow down brushes? What would be the hardware requirement for real-time out-of-gamut warnings?

The specification of my computer is written below.

CPU : AMD Athlon(tm) II X4 640 Processor
Mainboard : ASUS M5A97 R2.0
RAM : DDR3-1600 32GB ECC unbuffered (ECC is for ZFS)
GPU : AMD Radeon HD6450 1GB
OS : ArchLinux
Desktop Environment : XFCE 4.12 (I have considered moving to GNOME because GNOME supports cintiq much better.)

P.S. This computer used to be a file server, but after my desktop died, I haphazardly turned it into a file server and a desktop.