Ligatures are affecting scroll performance

I am using the FiraCode font with macligatures set. When scrolling down, MacVim seems to hiccup a bit, but when I scroll up it is very smooth. When I do not set macligatures it scrolls fine (but doesn't show the cool characters). I am testing the scroll speed using both my touchpad and jk keys and my key repeat is set at 1 which is 15ms.

This was originally posted in FiraCode's issues, but the solution to the problem probably lies in improving macligatures support.

OSX 10.11, MacbookPro Retina

Followed the instructions to

set macligatures
set guifont=Fira\ Code:h12

Doing some dumb testing against Activity Monitor, it looks like the CPU load when I scroll down is much lower (~%60) than when I scroll up (150-200%), so the downscrolling seems to be inhibiting the scroll rather than taking on a huge CPU load and causing framerate hiccups.

Looking around for some possibly related topics:
Guy with the same issue (Scroll to Amadeus Demarzi)
thread related to listchars

首先在 macOS 上使用 Fira Code 和 MacVim 重现该报告，使用 set macligatures 以及列出的 guifont 设置，然后比较启用和不启用连字时向上和向下滚动的情况。阅读链接的 FiraCode issue 和相关的 listchars thread 以了解背景；当启用 macligatures 后滚动不再出现报告中的方向性变慢现象时，即表示完成。