No Internet connection ever performs at 100%. There is always some loss, and the faster the connection is, the greater that loss is going to be. Different factors may affect the speed of the connection, such as the amount of traffic on the site or server the user is connecting to. Outside temperature affecting the cable poles making the RF/AC fluctuate due to resistance. As well as your modem/router can be causing issues if it is not receiving all the configuration files needed.

For information on how to check for reasons behind a slow internet connection refer to:

Checking For Packet Loss