如何使R渲染图更快

We are using R to spit out plots(heatmaps) which are being rendered on a shiny app (web page). Currently we are facing an issue with the time it takes R to render a plot taking out the time it takes to do the computation. Let me show the same through a contrived example. In this basic test case R takes ~17 seconds to render and save a heatmap file as png (data computer time is taken out : row and cols clusters are precomputed)We are using R to spit out plots(heatmaps) whic