I have not personally experienced any throttling or speed reduction with Fizz.
However it seems some customers have posted about their IPTV no longer working. In those case, I believe the issue may be related to the IP address they are using. Some of the Fizz IP addresses are new and not yet registered as Canadian. So the IPTV service may think the user is in USA (or some unknonwn default) and blocks the user. Often the suggestion give nto the user is to reboot the modem in hopes of getting a different IP address.
There is also the possibility the IPTV service has blocked certain IP address due to past breaking of terms of service agreement.