Parallel commands with Marionette

I made a script to take screenshot of pages using Marionette ("WebDriver:Navigate" then "WebDriver:TakeScreenshot"), but I have a few thousands of screenshots to take and it's taking a while. Hence my question: is there any way to parallelize this? I'd run multiple instances but I don't have enough ram on my server. I was thinking using sessions but it doesn't seem to work the way I thought it would...