Fusion repeatedly renders unaltered parts of video

I mainly use Fusion for pretty amateur purposes such as adding text to videos, usually with a tracker. I've got a laptop GTX 1050ti NVidia graphics card (4gb) and i7 processor (16gb). I would expect this would be enough for some simple rendering. However, with just a 30 second 60fps clip I get unbearable amounts of seemingly pointless re-rendering. I'll play through a yet unedited 10 second part of the video just to see what needs to be done at which point- Fusion will go through it slowly at first then play at 60fps which I'm fine with. I'll then do something as simple as changing the font of a text+ function earlier on in the video (only showing for about 3 seconds anyway) and I'll have to "re-process" the whole 30 second clip before I can do any more work. This happens with every minor change I make. The whole video will need to be processed again. Even if I DON'T make any change it's still unpredictable if I want to re-watch whether I'll have to wait yet again for it to process it at a slower fps before showing me the original. The dots on the nodes that usually show which viewer they are shown in start flashing crazily. It all has a very glitch feel. I understand if I have things shown in the viewer over the base clip like trackers or transform tools it will slow down but I make sure I always click off these. I'm not sure what I can do, whether it's a bug or my computer. I'm at the point it's so laggy I can't do any more work beyond a certain point. I've set my computer to always use the discrete graphics card but on task manager it's always 5% usage, maybe I don't have enough storage allocated to it? (new computer so have most memory still available)
Thanks in advance for any suggestions

What kind of video is your source material? Fusion is traditionally used with File Sequences, since most compressed video formats don't actually have seperate frames, they need to 'reconstruct' the image you're trying to watch from previous frames. This could be part of the problem you're having.

What kind of video is your source material? Fusion is traditionally used with File Sequences, since most compressed video formats don't actually have seperate frames, they need to 'reconstruct' the image you're trying to watch from previous frames. This could be part of the problem you're having.

thanks for the reply. The video is an mp4. What type of video should I ideally be using?

Mp4's are a heavily compressed format, which means your cpu will have to do a lot of work everytime you want to play it back. You'll want to use something like a prores file or ideally an uncompressed image sequence if your storage can handle it.