If it's the buffer, would not I see this cycle (Fast then slow) every time I use drive? The pattern I am seeing is that the drive is fast all the time for the first several months of me using it. When it does eventually get slow, it is slow from the beginning of every session. I have seen this...
Every single USB jump drive I ever bought, from cheap to premium eventually slowed down to the point of being unusable for large transfers. Most USB 3.0 drives start at ~100+/-50MB/s transfer speeds. As time goes by they work slower and slower. The slow downs are intermittent. Sometimes you get...