M1 Macbook Pro, external display; low refresh rate on HDMI, low DPI on Display port

I recently encountered a very annoying issue with LG monitor - and it happened to be M1 machine (well documented).

I think #1 and #2 might worth a try:

1.Firmware update -- by going to App store to download LG screen manager. From there, to check/update firmware of the monitor: I connected my LG 5K via TB cable, behind a TB hub - you might either need UCB-C or TB connection to establish the communication between Mac and display.

2.Reset the display setting by removing few .plist files associated with monitor via terminal --

sudo rm /Library/Preferences/com.apple.windowserver.displays.plist

Reboot will be required after.

Best

LG – 45 Inch UltraGear™ OLED WQHD 1440P 240Hz 0.03ms G-Sync Compatible 800R Curved Gaming Monitor

That's a 45", 3440x1440 monitor with a refresh rate of 240 Hz and a pixel density of only 84 PPI (according to LG – I calculate a slightly lower figure). That's lower than the pixel density of a 24" 1920x1080 monitor (about 92 PPI) or a 27" 2560x1440 monitor (about 109 PPI)., so text and icons may be a bit coarser than on those monitors.

I would guess that the USB-C to DisplayPort adapter cable is using DisplayPort Stream Compression and that 3440 by 1440 pixels @ 240 Hz may be forcing it to do so much lossy compression that you are seeing visible effects. But this is just a guess, based in part on the report that things work @ 60 Hz.

That's like a 4K display with the bottom half missing

with HDMI 2.1 and DisplayPort 1.4 inputs.

Your MacBook Pro with M1 does not support HDMI 2.1 on its Direct HDMI Port, so the Direct HDMI Port may not be adequate because its maximum supports 4K at 60 Hz.

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USB-C ADAPTERS typically peter out at 4K, often at 60 Hz.

To get good higher-than-4K@60 display bandwidth, you typically need to use a ThunderBolt (not just USB-C) Dock that features Internal conversion Direct to HDMI 2.1 or Direct to DisplayPort 1.4.

As currently implemented, that Dock makes your displays situation Worse, because it does not do any of those conversions internally. Your Dock provides only ThunderBolt outputs, and you are forced back to using USB-C adapters.

As Servant of Cats suggests, USB-C adapters might work up to about 60Hz, but must use compression that may be more lossy for DisplayPort at higher resolutions and refresh rates.

Punchline:

The right solution requires converting ThunderBolt (twice the bandwidth of USB-C) directly, and that is only available inside a different ThunderBolt Dock.

Only for M2 and higher Macs, a direct connection using a certified ULTRA HDMI cable from the Built-in HDMI 2.1 port on the computer would work great. HDMI 2.1 built-in is what makes higher resolutions available.