One thing to remember is that SD cards use flash memory and they are not designed to be written to indefinitely. The controller built into flash memory devices manage where the data is written so that some memory locations are not written to all the time while other locations are avoided -- that would result in an early death for the device. So, they manage this by spreading the data around to avoid over using some locations. The consequence of this can be a reduction in performance as the controller writes some data here and other data there to more fully exercise the totality of storage.
So, what I'm saying is that it IS possible for an SD card to degrade in performance with time. The controller should even this out, but there can be times when writing data the device is slowed by the need to skip areas that are available but have been used more than other areas.
Lastly, I think the way DJI handles SD cards, or more precisely uSD cards, is not as tolerant as I'd like and I think it is this very issue that gives me the image tearing problems I've described numerous times in my videos. In this video, at about 3:13+, I show this tearing a couple times.
I use a top quality Sandisk Extreme Plus 128GB SDXC V30 U3 memory card and have also used a Samsung Evo 64GB SDXC U3 memory card with the same results -- tearing in many videos. On some flight I have no tearing issues at all and on other flights I can have dozens. My suspicion is that the
P4P is not quite capable of handling the volume of data and the work to be done on it and the controller gets behind and begins to drop frames. I think my settings of UHD@30 with D-Cinelike, -1/0/-1 and H.265 may put extra workload on the controller and for that reason I see issues that some other do not.
If the card has to jump all over the place to write data to avoid overusing some locations that could further exacerbate the marginal controller/processor in the
P4P and make the tearing issue more common.
Brian