Don't think the speed is an issue, more like delivery of content over your connection in your area is being interrupted somehow.
I'm on VF Gigabit and my VFTV is consistently allocated from 200 - 300MB via the Ultrahub on Ethernet (5M of Cat-6 cable) while streaming content at 1080p.
However, if you are not connecting to VFTV from modem via Ethernet, but via WiFi, then there may be other factors impacting your speeds, like kitchen whiteware (FF, microwave, stove), electrical wiring or water pipes close by, steel reinforcing, steel framing, or other embeded metal, cordless phones on the same WIFI frequency, wireless capable printers, even TVs can impact on WIFI connections.
And, Sagemcom uses a flavour of Linux for the VFTV Operating System.
I have found that their is a link between the Power Settings and Stability. However that has a fish hook.
The Boxes are more stable (we have 2) when you leave them on the Quick Start setting. However the catch is that its using large amounts of data every hour to allow quick start. (talking several GB per hour). Which as well as being wasteful makes the rest of the houses internet compromised.
I do now regret getting Vodafone TV and commiting to a contract. At this time its just not good enough. Try Amazon, Netflix or any other TV using a Kodi Box, Xbox or Playstation and you will know this is not good enough. To be honest I thought the Sky Digital Plus was a better experience.