Which environment are you using?
When creating a new project in GWD you are able to select an environment. By default is DoubleClick.
This adds additional file (in the case of DoubleClick, the Enabler library).
If you open your Chrome console, while running the banner, you will see that the banner is not initialized until the enabler is available.
You will also see how much time it takes to load the enabler.
If your colleague is creating a banner without that library, or without correctly listening to the event Enabler.initialized, this may be the main reason for the discrepancy.
If you don't want to include this additional library (that is used to integrate your banner in DoubleClick Studio), just select Generic from the environments dropdown.
There is also another reason that may cause the delay, and is the PoliteLoader.
You can select to politeLoad the banner from the Publish menu.
If the PoliteLoader is selected this cause the banner to be initialized only after the page is fully loaded. This may cause delays compared to a non polite loaded ads.