Lag on 4k external monitor and MacBook Pro (early 2016, internal graphics) running in retina mode: can bad cable be a reason or just not enough power?

I just bought new monitor (LG 24UD58-B) because of my MacBook's flexgate issue (when you can't really open lead). I can't bring it to service yet, because where I live it takes up to 5 weeks and it's my working computer.

For some reason I experience very laggy behavior while running in retina mode, it's definitely not 60 fps, probably closer to 25-30. It seems that the problem is in CPU spikes because other movements like moving the cursor and scrolling are fine sometimes. It is happening in all applications.

I'm using default hidpi mode, so no additional scaling is present. Laptop is working with closed lid. System is High Sierra.

Currently it is connected it with noname usb-c → displayport cable (the only I could get in my country).

Could the actual cable be the reason of the lag / slow performance?

Can I check somehow whether it's CPU vs cable? Because from apple site it seems that my model should handle 4k just fine.

The problem is I can't really check with another cable unless I order another one from amazon which ship around two weeks. I would probably better to return the monitor before that. I can just buy 1080p monitor for half of the price then.

I also tried running with HDMI adapter and it works pretty smooth in low resolution mode. I have small lag with the current cable even in low-resolution mode.

So, if I read your question, and understood it correctly, you are using a USB-C cable to connect your screen to your machine?
The thing is that a USB-C type cable is not the other!
There are a lot of differences in USB-C cables so you have to be sure that your cable supports what you want it to do.
If the cable's bandwidth is not sufficient for supporting 4K it can result in lags as you explain.