If you do not have unlimited Internet then you will be billed for overage but if you do have unlimited then you do not need to worry about overage fees.
The answer to your question would be Fido can throttle your speed.
According to the Acceptable Use Policy taken from here.
We reserve the right to manage our networks (or third party networks for which we have roaming or network sharing agreements) in order to optimize their efficiency for the benefit of our subscribers, including, without limitation, by way of the following: rate limiting (speed), rejection or removal of spam or otherwise unsolicited bulk e-mail, anti-virus mechanisms, and protocol filtering. We may take any other action we deem appropriate in order to help ensure the integrity of the network experience for all subscribers. For details on our network management practices, visit fido.ca/networkpolicy.
Welcome to the community!
Are you noticing a specific reduction in speeds? Are you connected via Wifi or directly to the modem? I assume you're referring to Unlimited plans.
While I cannot provide a definitive answer, I doubt they throttle speeds once you reach a certain cap. The main reason to throttle speeds is to free up bandwidth for other users -- often referred to as Fair Usage. Large bandwidth users can be a real problem for mobile data networks because mobile bandwidth is limited. There is a physical limitation as to how much data can be sent via the airwaves. On the other hand, much more data can be sent via cable (and faster too) so having to free up bandwidth isn't as big an issue.
In addition, if they did throttle speeds once a certain cap had been reached, they would have to indicate the amount at which it gets throttled. For example, when tethering Data Bytes, maximum speeds will be reduced to 3.8 Mbps for both download and upload (see here). I have not found any indication that speeds get throttled for Home Internet anywhere on the website. Admittedly, that does not mean it isn't written somewhere I haven't found yet.
Hope this helps