When using the basic attachment mode, features like the animated progress bars,will be disabled and also you won't be able to attach multiple files at once. Though limited, the basic mode works perfectly when having a slow internet connection.
To enable this settings:
Sign-in to your Gmail Account.
Click the gear button located in the upper right corner (Options > Mail Settings).
In the "General tab" , scroll to the "Attachments" section and select "Basic attachment features"