Update versions and tweak a bit python language server config.
After some investigation, I understood that completions where slow for us because jedi add some buildout support which walks up directories to find a buildout.cfg and inspects the corresponding scripts in bin to find python package paths. On web runner I have 98 scripts in ~/bin/, each of them adding between 30 and 50 entries to sys.path.
A quick workaround is to create an empty buildout.cfg in the current directory or a parent directory before editing python code.