App unusably slow after upgrading to 1.4

I’m developing an app that handles over 55000 records, it worked well under 1.3, however after migrating to 1.4 meteor itself has become incredibly slow. When running the app it hangs on the => Started proxy. message for over 10 minutes, and the accounts-ui loginButton template even takes a few seconds to load!

I rebuilt the npm packages like the migration guide suggests, and even deleted and recreated home/.meteor

When I switch away from the live data into using a few records of sample data, it works fine. What is breaking my app?