Lots of missing information here.
- What speed tier are you on?
- What speeds are you getting?
- Wired or wireless connection?
- Why do you want to delete devices from the router? Their presence won't affect your speed.
- How many STBs do you actually have? You shouldn't delete them since they need an internet connection for VOD and guide data.
BTW, the MI424-WR is a router, not a modem.