Performance: with two Macs, a stronger Mac slows down during a video capture, but a weaker Mac does not

I've been to two MacBooks:
(1) MacBook Air, 11 inches, 2013, with 10.11.6, 8GB of RAM, Intel Core i7 at 1.7GHz, integrated graphics
(2) MacBook Pro, 15 inches, 2018, with 10.13.6, 32GB RAM, Intel Core i9 at 2.9 GHz, Intel UHD Graphics 360 and Radeon Pro 560X

I run two screen capture programs simultaneously. every on both Macs. Used to:
(A) Screen registration utility:
(B) A custom screen registration utility originally compiled for macOS 10.11
So the same two programs in every of MacBook Air and MacBook Pro.

My MacBook Pro (the apparently strongest machine) slowed down significantly after a while, while my MacBook Air did not. By slowdown, I mean:
Animations (eg, Opening a window or clicking on an icon or scrolling through a terminal window) in the GUI of my macOS become choppy and are not smooth, and it takes a while to respond to a click or by pressing a key. This happens throughout the operating system, and is not specific to any application. That is, when I click on the Dock, System Preferences, etc., everything is slow.

I'm trying to understand why. I suspect that it has to do with the fact that the screen capture software (B) was not compiled for macOS 10.13. I'm in the correct way?
If yes, why not compile for the current version of the current operating system as a result a deceleration like this? I hope to get an answer from the perspective of a computer architecture / "under the hood" / operating system, for example. Does it have to do with RAM, disk writes, GPUs, CPU interrupts, kernel system calls, swapping, paging, etc., and how exactly?