set-window-margins: lag moving cursor?

[Marcelo Toledo]

Hi,

I've recently noticed a lag moving cursor around. When trying to track down the 
problem I’ve noticed that it was related to set-window-margins.

Why am I using set-window-margins? 

When I use really large monitors with GNU Emacs and only one window, I like to 
add margins left and right to align 100 columns of total width in the middle. 
Code gets centralized and it’s easier to code.

Code I am using currently: https://github.com/marcelotoledo/centralized-mode
Code I was using before: https://github.com/rnkn/olivetti

What environment I’ve tested:

* Mac OS X 10.14
* GNU Emacs 26.1 (latest stable version)
* Build options: ./configure --build=x86_64-apple-darwin --without-dbus 
--with-ns

In that environment I’ve tested speed of cursor moving around random files. 

How exactly I’ve tested? 

I’ve set the cursor in the top of the buffer and measured the speed to reach 
the buttom simply pressing the key binded to <down>. I’ve done that several 
times and calculated the average (it was pretty steady).

When I don’t have set-window-margins set it’s moving in a regular speed.

But whenever have set-window-margins set, it’s easy to notice a ~ 25% reduction 
of speed moving cursor around.

I’ve been reading the source code and trying to figure out what exactly is the 
problem and I was wondering if anyone could help me understand why this is 
happening and what part of code is bringing this behavior.

Happy to provide more data.

Thanks in advance