Form refresh performance

Sometimes when I run my Vaadin application it is slow and the blue progress bar runs across the top of the screen with every single action. Other times (the minority) it is extremely fast. I get the same outcome in both Chrome and Firefox.
Intrigingly, when I uploaded a Vaadin application to a server and asked colleagues to test it, the application ran fast for them and slow for me! I have tried accessing the Vaadin application from more than one PC (in case it was an issue with my dev PC) but get the same result.
What might trigger the blue progress bar to run all of the time and why might different users get a different experience?
Help and guidance would be much appreciated.
Kind regards