Nobody likes a slow running computer, specifically
If your computer is operating slowly, it can be for a lot of reasons, so doing the necessary research beforehand may be crucial and end up saving you a bit of effort and money when it comes to exploring a solution. If you find that it's a software issue, you can likely just download more current drivers and fix the issue that way, but if it is a hardware issue, there are a couple things you could do to improve your unit's performance.
Letting you send and receive data at a faster rate, the simplest solution may be to just purchase a new router or modem with a higher signal output, but there are also some other options for those who're open to getting a little more up close and personal with their unit. You would get a smoother, quicker connection with less lag and a more steady data transfer rate allowing you to watch video and work a lot faster than you were before by upgrading to any more advanced Cisco SFP modules.
You could even go a step farther than that by exploring other transceivers that exceed the performance specifications established by SFP transceivers for those that are more aggressive when it comes to their unit's performance. You might install any Cisco GLC T module in its place, for instance, with both parts being of corresponding dimensions as well as being swappable hot form pluggable and increasingly more reliable than their counterparts. Supplying a flow of data stronger than you're probably experiencing presently, there transceivers can function at a rate of ten, one hundred, or one thousand Mbps speeds over up to one hundred meters of unshielded twisted-pair Ethernet cable.
These are only a few of the choices available to you to increase your device's performance, but to get a better idea of what you could do, find where your current issue lies, and then do the proper research needed to find out if it's something you could take care of on your own; the results may surprise you.