It has been easiest for me to connect my iPhone to my PC and transfer the photos from within the DCIM folder onto my PC’s hard drive. From there, I can quickly drag them all to iNat’s uploader, with all relevant metadata intact. I’ve done this with up to 300 photos (~200 observations) at once, and sometimes the larger volumes do slow things down a bit, but it’s still way faster than doing them individually.
I have an iPhone, but the transfer to PC works fine. I imagine it should work well with Androids as well. I would also think that this would be quicker than downloading from the cloud!