This is my solution for the problem described in the old thread Dual Monitors Makes Computer Run Slow that I found when searching for speed issues when working with dual monitors on Linux Mint / Cinnamon. I had a perfectly functional dual monitor setup on Windows 7 and yet was plagued with the same issue when to switched Mint so I didn't buy in the slow hardware argument and indeed I found out that by setting a lower number of default workspaces significantly improved the situation. This should be the first step: I went from the default 4 workspaces to 1 and the situation was pretty much ok though not perfectly seamless. I did this months ago so you'll have to look around for the exact command to use. Also, I chanced upon a "Only use workspaces on primary monitor" option in the Workspaces GUI app today that may achieve the same effect without sacrificing the use of workspaces entirely.
Just today I achieved a perfectly seamless result on my computer by working directly with xrandr after reading something on the limitations of mixed DPI screens (http://wok.oblomov.eu/tecnologia/mixed- ... /#index7h3). Very simply, I virtually scaled my secondary display down to the exact same resolution as before:
xrandr --output DVI-I-1-1 --scale 1x1.001 --mode 1920x1080 --rate 60
It's a dirty hack and I barely understand why it works but it definitely does on my system so I wanted to share it to help others in the same situation