I’ve not noticed rendering being particularly slow but I actually wrote a rule to do exactly what you asked for, pre-generate the static images from a rule and have the sitemap just load the image from there anyway, mostly because I didn’t want to have to generate the chart every few seconds if I’m looking at the last month’s worth of data.
Near the end of the original post there is a link to Grafana Image Charts where I posted the rule and how it works. There is Rules DSL, Rules DSL with Expire, and a Jython version posted.