https://github.com/veluca93/fpnge is a very fast png encoder. A bit lower compression ratio, but runs significantly faster than alternatives. Here is a presentation with benchmarks:
This worked out for me. What I was actually interested in was reducing CPU utilization, which generally speed is a fine substitute for (the same work being done in a smaller time slice means lower overall utilization). It reduced utilization enough that I'll likely push to use it for production in the future (not immediately, there's bigger low hanging fruit).