There could be several factors. How powerful is your computer? What other software have you installed on it since you started? Are you plugging the hardware into the same ports every time you stream? Have you raised the resolutions on the cams without realizing it? Are you using more stream overlays now or embedded chat apps (all these eat into your computer's resources/memory)? How many times have you updated the PC since (Windows particularly if that's what you use)? Lots of things can affect the performance over time, especially if your computer wasn't fully up to snuff to begin with.
Case in point, when I first got into pinball streaming, I used a cheap laptop that was barely suitable for the task, and while the stream was passable at first, it become unwatchable as time went on despite using the exact same hardware and only installing the usual Windows updates to it. I still had the computer running about as lean as it could and it wasn't used for anything other than streaming.
*Edit: I checked out your stream. I'd try lowering the resolution on the playfield camera a bit. It looks really sharp which makes me think you've got it fairly high. Take it down to 1280 x 720, and lower if that's not working, then see how your performance works from there. You can get away with a lower resolution on the playfield cam because it's not taking up the whole screen. Also, 1080p feeds from the cam can bog down the USB bus pretty quickly. Another troubleshooting method is to switch the cams around on your USB ports. Also, is that a Mac, or a PC? I couldn't tell in the background.
As an aside, make sure you have auto-focus disabled, I noticed that kicking in a couple of times. That probably won't effect the smoothness of the performance, but it will make things blurry randomly as it thinks it needs to re-adjust.