Your development machine is usually on a very good network connection.
To test how your application behaves on a slow network (e.g. mobile), you can simulate limited bandwidth.
There is a Linux utility called "trickle" that allows you to do limit the bandwidth available to a process.
You may need to install it first. For example, on Ubuntu:
sudo apt-get install trickle
To start an application with limited downstream or upstream, run it through trickle. Example:
trickle -d 50 -u 20 firefox
This will start a Firefox process which is limited to 50 kB/s download and 20 kB/s upload.
Note that this will not work on some applications (e.g. Chrome).
Note that mobile devices often also have limited CPU resources. We have a separate card about that.