The actual speed that your SB6121 is receiving is largely dictated by your service provider, or how clean the signal levels are on the coax line coming into your home. You can add a mAX mesh system to your SB6121, and this can help to increase the speed within your local area network. It would do this making sure that there is ample Wi-Fi coverage in your home. 

What speeds do you pay for from your isp? 

What speed do you get wired into the sb6121?

those are kind of key as @Where the Wild Pings Are says, the limit is what you’re provider is supplying you with. If you’re area is only capable of 100mbps connections, the sb6141 is fine. But if you’re paying for more speeds or your ISP is changing the speed requirements that require docsis 3.1 device, it might be worth upgrading.