So my results surprised me a lot. My GPU’s best time was around 9 minutes. My CPU was giving me better results and so I first tried to go as small as possible. The smaller I went, the closer I got to a 6 minute render. Then I tried going bigger and bigger, and I was getting faster results. My fastest render time was only 4 minutes and it was at 400, 400 tiles. Above 400 by 400, it started slowing down significantly. So my advice is that on CPU, bigger can sometimes be faster!
UPDATE: The big tiles would render fast at first and I was going off approximate time. But then it would slow down significantly. Now there’s a new lesson to be learned: Don’t go off approximate times!