U2720Q, MacBook Pro, 29Hz?

Hi

I've this Macbook Pro:

MacBook Pro (15-inch, 2018) with macOS Catalina 10.15.4
CPU 2,6 GHz Intel Core i7 6 core
RAM 16 GB 2400 MHz DDR4
GPU:
Radeon Pro 560X 4 GB
Intel UHD Graphics 630 1536 MB

that I connect to my new Dell U2720Q (just arrived some days ago) with the U2720Q USB Type-C cable (C to C).

I rescaled it to 2560 x 1440

I noticed that randomly it's pretty slow. I especially noticed when scrolling webpages, moving cursor, dragging windows. Working all the day on that I'd say it's close to unusable.

From Apple menu > About This Mac > System report > Hardware > Graphics/Displays I see:

• Resolution: 5120 x 2880 (5K UHD - Ultra High Definition Plus) ==> why 5k? the monitor is 4k
• Appearance user interface: 2560 x 1440 @ 29 Hz ==> correct chosen resolution but strange Hz... why?
  

Then I tried a USB-C-->DisplayPort cable, keeping the rescale to 2560 x 1440 and it's a lot faster, smooth, I think it's how it should be. But, from System report now I have:

• Resolution: 3840 x 2160 (2160p/4K UHD 1 - Ultra High Definition) ==> these seems correct, right? it's a 4k monitor
• Appearance user interface: 1920 x 1080 @ 60 Hz ==> why 1920x1080? I've chosen 2560x1440, indeed the visible resolution is like before, not smaller...

Then, after sometime I rechecked the System info and...

Randomly. I don't understand anymore.

Also, I will need to use the USB Type-C cable (C to C) for the charging feature (PD port).

Please, someone have never seen this thing? I need to solve as soon as possible

Thanks