High CPU loads (100% on one core) and slow response with second monitor plugged in VGA port

• Cinnamon version (cinnamon --version)

$ cinnamon --version
Cinnamon 4.4.8

• Please specify if you are using the daily builds PPA
  (https://launchpad.net/~linuxmint-daily-build-team/+archive/ubuntu/daily-builds).
  I'm not using daily builds or any other PPAs

• Distribution - (Mint 17.2, Arch, Fedora 25, etc...)

$ lsb_release --all
No LSB modules are available.
Distributor ID:	LinuxMint
Description:	Linux Mint 19.3 Tricia
Release:	19.3
Codename:	tricia

• Graphics hardware and driver used

$ inxi -G
Graphics:  Device-1: Intel Mobile 945GSE Express Integrated Graphics driver: i915 v: kernel 
           Display: x11 server: X.Org 1.20.5 driver: intel unloaded: fbdev,modesetting,vesa 
           resolution: 1024x600~60Hz, 1400x1050~60Hz 
           OpenGL: renderer: Mesa DRI Intel 945GME x86/MMX/SSE2 v: 1.4 Mesa 19.2.1 

• 32 or 64 bit
  32 bit

• Attach /home//.xsession-errors, or /var/log/syslog
  No errors are logged. I've attached a screen capture of sysprof window showing issue.

• Issue
  When I plug a second monitor into the VGA port on this machine (EeePC 1005HA netbook), I get high CPU loads, 100% on one core of the dual core processor, and the system becomes extremely slow and unresponsive. If I unplug the external monitor, the high CPU load subsides and the system is responsive and performs as expected.

• Steps to reproduce
  Plug in external monitor, issues occur 100% of the time. Unplug the external monitor, and the issues go away.

• Expected behaviour
  I expect performance to be about the same with a second external monitor attached as it is with only the integrated laptop display.

• Other information
  This happened after some recent updates. I wish I could be specific, but I've been traveling (and not using an external monitor) for about 4 weeks, and during that time many updates were applied. I do see new xorg server updates, and I suspect those are the updates that triggered the issue. I also upgraded from Linux Mint 19.2 (Tina) to Linux Mint 19.3 (Tricia) during that time, so as I stated, a lot of things were upgraded.
  Issue occurs regardless of kernel used. I've had the issue with LTS kernel version 4.15.0-74, with kernel 5.3.0-26 or with kernel 5.0.0-37.
  When the issues are happening, the "top" command run in a terminal window shows "cinnamon --restart" as the command that has 100 in the "%CPU" column. The same issues occur in a XFCE session, but in XFCE, the process that shows "100%" in the %CPU column in top is "xfwm4." I've also reported this issue to the xfce4 developers/maintainers over at xfce.org. The person who responded over there asked me to run "sysprof" to "see where it's spending time" when the issues occur. Sysprof shows "in file /usr/lib/i386-linux-gnu/dri/i915_dri.so" as the biggest CPU hog. That is common to both "xfwm4" and "cinnamon --restart", depending on which session type I select when I log in. I've attached a screen shot of sysprof with the capture from Cinnamon.
  Since this may well be an issue with the i915_dri.so module/library, I'll be reporting this on Ubuntu's bug tracking through their website as well (as an issue with the libgl1-mesa-dri package).
  

• Known workaround (for XFCE)
  The issue does not occur if "Enable display compositing" is unchecked on the "Compositor" tab of the "Window Manager Tweaks" settings in the XFCE Settings. I point this out because there may be a similar setting that can be used as a workaround in Cinnamon.

Reproduce the slowdown with a VGA monitor attached, then inspect the sysprof capture and the /usr/lib/i386-linux-gnu/dri/i915_dri.so path identified in the report. Compare Cinnamon and XFCE sessions, including XFCE with display compositing disabled, and use the reported kernel and graphics-driver details to narrow the component involved. Done means the responsible regression is identified and a verified fix or upstream handoff is documented.