You can fix this problem by simply turning your mobile's Airplane mood on and off.
If that process doesn't work for you, then you should log into your Snapchat account for a few seconds. Log in to your account and see if it solves your problem.
Another thing way you can help yourself to overcome the slowness of Snapchat is to restart your phone. Sometimes, the issue is too little and will be solved when you turn off the mobile and restart it again.
Despite doing all the above steps, if you see no improvement in Snapchat functionality, you should check your device's internet connection. Sometimes, the internet speed is so slow, which causes Snapchat to function slowly.