Speeding Up Ubuntu 19.04 in Dual Monitor with Virtualbox

I'm using Ubuntu 19.04 (Lubuntu 19.04, in my case) with Virtualbox. If I don't have my second monitor connected to my laptop, the GUI transition animations (hover over menus, etc.) in GTK2, GTK3, Qt, and other widget libraries load very fast and my mouse doesn't jam. However, the moment I connect my second monitor and tell Virtualbox to load that as the primary monitor, the transition animations drag and the mouse occasionally jams until the transition animation caches, and even then, it caches only for so long and jams again while it rebuilds the transition animation cache. This is especially evident in Chromium browser and my Pluma editor, but not so much at all with the Q-based apps that come with the new Lubuntu 19.04.

Some articles have suggested that I could attempt xorg.conf tweaks. However, that file no longer exists and I only have /etc/X11/xorg.conf.d, and in that folder there's only a keyboard conf file, nothing more.

Why is it that when booting Virtualbox display into my laptop is fast, but booting Virtualbox display into my attached monitor is slow? Note that this problem didn't occur in Lubuntu 14.04 32 bit, occurred slightly but nowhere near as bad in Lubuntu 16.04 32 bit, and now occurs a lot in Lubuntu 19.04 64 bit. (They don't make a Lubuntu 19.04 32 bit.)

I'm running a Mac Air 13" (mid 2013) with an attached ASUS (Make:MX279/Model:MX279H) 27" display. My Mac tells me that my graphics chipset is Intel HD Graphics 5000 1536 MB graphics.

Note, I don't mean to imply here that I'm trying to boot Linux into a dual monitor setup. No, my Mac OS is, but my Linux I want to boot into my attached monitor, not in dual monitor config.