Do I need an external GPU?

2017 Macbook Pro 15"

Simultaneously supports full native resolution on the built-in display at millions of colors and:

• Up to two displays with 5120-by-2880 resolution at 60Hz at over a billion colors
• Up to four displays with 4096-by-2304 resolution at 60Hz at millions of colors
• Up to four displays with 3840-by-2160 resolution at 60Hz at over a billion colors

Thunderbolt 3 digital video output

• Native DisplayPort output over USB‑C
• VGA, HDMI, and Thunderbolt 2 output supported using adapters (sold separately)

GKK999 wrote:

I am going to buy two monitors for my MacBook Pro (2017 15-inch Touch Bar), from 27 inch to 34 inch each. I was thinking if my built in GPU would be strong enough to manage these two monitors and the MacBook screen at the same time or if it would be a good idea to buy an external GPU?

You can read more here:

Use an external graphics processor with your Mac

duet and similar solutions are based on creating a software display buffer in RAM, and using a more mundane interface such as USB-2 to send the data out to a stunt-box that puts it onto a "real" display cable.

They are slow because the display buffer is simulated in main memory, there is no graphics processor, the link to the stunt-box is slow.

They are often running DisplayLink software.

When you have a Hardware-supported interface, it has GPU Hardware acceleration for drawing and shading, a Hardware rasterizer to produce the screen data, and a high-speed direct link to the display.

There will be no discernible load on the CPU or the rest of your computer in refreshing that display. The lag is gone.