We are observing slow downloads and errors using yarn or npm, this may impact and slow down deployments of node applications with lots of packages especially.
This seems to be an issue on npm side, and does not seem to be restricted to Clever Cloud.
Update: NPM Registry posted on their status page confirming the incident and are working on a fix: https://status.npmjs.org/incidents/bydjtj102gsn
Update: The issue has now been fixed, node deployments are back to normal.