Why does MacOS slow down terribly when switching to 3008x1692 resolution on an external display?

[MapleGreen]

2160P HiDPI means rendering at 7680x4320, did you check that?
View attachment 853366

What you see in the system preferences is the UI Looks like resolution. The actual rendering resolution need to be checked at system info.

in 3840x2160

in 3008x1692

but I'm not getting blurry image in 2160p it's Cristal clear
but in every other low resolution modes I get blurry screen

look how in 3840x2160 my monitor listed as it should be (2160p 4k UHD ... )
but in 3008x1692 it is not listed

 

[MapleGreen]

HiDPI 3840x2160 means rendering at 7680x4320, you tried that setting?

I assumed that whatever we choose does NOT actually change the resolution of the display, but instead scales up only the text on the display so that it "looks like" the numbers you chose under scaled. When text is drawn, it is doubled or tripled or times 1.25 or whatever it needs to be shown properly.
but in this case it actually changes resolution, my desktop background quality changes as well!

in my macbook pro I get different tabs in System Preferences/Displays

 

[h9826790]

in 3840x2160

View attachment 853381

in 3008x1692
View attachment 853378

but I'm not getting blurry image in 2160p it's Cristal clear
but in every other low resolution modes I get blurry screen

look how in 3840x2160 my monitor listed as it should be (2160p 4k UHD ... )
but in 3008x1692 it is not listed

Anything below 2160P, and not in HiDPI resolution, will looks blurry, that's normal.

When choose 2160P, that's the monitor's native resolution. Won't be in HiDPI, but won't looks blurry, that's also normal.

When you choose anything between 1080P and 2160P in HiDPI, the screen will render at higher than 3840x2160, then down scale back to 3840x2160, and transmit to the monitor. So, it won't looks blurry, but more demanding to the GPU.

 

[MapleGreen]

Anything below 2160P, and not in HiDPI resolution, will looks blurry, that's normal.

When choose 2160P, that's the monitor's native resolution. Won't be in HiDPI, but won't looks blurry, that's also normal.

When you choose anything between 1080P and 2160P in HiDPI, the screen will render at higher than 3840x2160, then down scale back to 3840x2160, and transmit to the monitor. So, it won't looks blurry, but more demanding to the GPU.

is there any way to fix this in this section by creating new custom profile?
/System/Library/Displays/Contents/Resources/Overrides/

so macOS treats my 4k monitor as native 1440p or 1692p

If it is not possible I think 1440p Native monitor performs better than a 4k monitor with scaled resolution to 1440p