Your internet speed is ultimately dependent on your network provider. Therefore, if low speed is becoming a common occurrence for you, you may want to consider a different provider.
The other thing to look into is the bandwidth that your mobile connection is using. If your network connection is anything less than LTE speed (ie. 4G, 3G, etc.), then you will have a slower connection. Also, make sure you are in an area where you can get a strong, consistent mobile signal. Together those factors should enable you to obtain faster, consistent internet speeds.