Analysis of output.xml on 200+ builds means a lot of cpu, which seems to be normal for me!
I see 3 possible solutions:
1. Did you consider to use parameter "Max builds" (e.g. set to 20)? Then the last 20 builds are shown only.
2. Did you consider to automatically discard old builds? You can configure amount of days of amount of builds to keep in project configuration.
3. Currently the image generation is on demand and starts, when user requested it via frontend. Robot-Plugin could precompute such images when the build is finish.