I’m tagging this as support, but I have already worked around the problem. Was just wondering if it deserves an issue on github.
My content/ folder is around 1.9 GB and last night I deleted the resources/ folder to generate the images again. It was taking forever and the hugo process was not taking up any memory or cpu. When I ran the command in verbose mode, it gave me this error message:
WARN 2018/06/10 16:23:12 WARNING: Timed out creating content for page "stories/museum-tour/2018-02-13-Museu-Nacional-Arte-Antiga/index.md" (.Content will be empty). This is most likely a circular shortcode content loop that should be fixed. If this is just a shortcode calling a slow remote service, try to set "timeout=20000" (or higher, value is in milliseconds) in config.toml.
I did set the timeout value, ran it again, and another page was giving out that error with the process also appearing to be stalled.
My guess was that generating images was taking too long because there were so many of them pending. Running Hugo a few times eventually made it build as expected.
Since then, Hugo is back to running smoothly.