I have a high speed SD card suitable for recording 4K video and recommended for use in cameras.

The ability to refresh 4681 caches in 2-3 minutes while I had cell phone data in a remote location before going back into the wilderness was awesome. I was able to get logs posted between the time I left home, and the time we found some of the caches we attempted.

I had NO issues with the app version I tested over the weekend.

Sweet! Looking forward to trying it out... 
Even more so if it's stable by the end of the day tomorrow!!! I'm going on a #GeocacheRoadTrip Friday morning. Should be up to 500 finds for a 3-day weekend

You don't need to get the responses right away if you use an asynchronous model with callbacks or promises. At least that's what I'd use in the Javascript world. Not sure what you can or can't do in an Android app.

I would happily beta test this for you - but not until June 10 as I need to use the app heavily between now and then! 

This would be a HUGE improvement over the 45+ minutes it took this morning.

I'm a web developer, not an Android developer.
I would probably handle the problem something like this in a web app

* Track the number of API calls made within the previous minute. Let this be numAPICalls
* When a user refreshes a database, build all the API calls necessary and drop them in a queue
* Send as many requests as you can (60 - numAPICalls) from the queue
* As requests complete, grab more requests from the queue and send them
* Continue until all the requests are completed.

My example of 4681 caches should be able to process in under 2 minutes. (60 calls per minute, 50 caches per call).

Would definitely be a lot faster!

Can you queue up the API calls at once and run them asynchronously instead of in sequence? 

So my update today would have been 94 requests of 50 each.
Is the API rate-limited (requests per minute)?

Can you make the number of points an option? Every 1s seems to be too much, but at highway speed (100km/h), that's nearly 28m between points. But at walking speeds, in 1s, I'm only going to move a couple of meters.

I would suggest every 6 seconds if you can't make it an option. 10 points per minute. For most purposes, that should be reasonably accurate - I think...?