Is it normal that PS starts lagging with this brush?

Hey guys

So Im working with an Wacom Intuos 5 on a digital painting (just started doing this) that I want to print out later on (1.20 x 1.20 meters) .

My file is pretty basic, only 5 or 6 layers and the file size is about 650MB. Resolution is 15.000x15.000 pixels @ 300 DPI and 8-bit ( maybe a bit overkill but i did this to test it out)

When I start painting with a basic soft round brush...i have no lag with the Wacom (or mouse)  and it seems to work fine.

However, as soon as i use a 'more specific' brush... Im getting lots of lag. For example...the default brush in photoshop called: Round Blunt Medium stiff.
This has no brush settings enabled, so its still pretty basic.. But already giving me tons of lag when I use it (with pen, but also just with mouse)..

My question is.. is this behavior normal? Because the canvas size is so big? Or am I missing something in the settings that I can change to fix this. Becuase im going to use a lot of specific brushes (watercolor brushes) that I bought.. and they also have this problem which makes them unusable for this size...

I have a powerfull PC, so shouldnt it just work without lag?

this is what I did so far to solve this

- seperate scratch disk
- smoothness to 0- turn of my second monitor

- set 'history and cache' to huge pixel dimensions
- GPU drivers are updated

This is my PC setup

CPU: Ryzen threadripper 2950X (16 cores)

Motherboard: X399 DESIGNARE EX

Ram: GSkills 64GB

SSD/HDD: 3x Samsung Evo 970 -  1TB / 3x WD HDD 2TB (raid)

GPU: GeForce GTX1080Ti

PSU: corsair HX1000

Chassis: Fractal Design Define R6

OS: Windows Home 10 - 64 Bit

any pointers are appreciated