Thanks Andy.
We're just zooming around on the map - they are query requests from what I can see. Here's one of the requests (I replaced the actual layer name with "OurFeatureLayerName").
https://services.arcgis.com/eP8V7tJrYd6ZUBkc/arcgis/rest/services/OurFeatureLayerName/FeatureServer/...
1) Here are the query string parameters:
f: json
geometry: %7B%22xmin%22%3A-9223809.077229367%2C%22ymin%22%3A4852834.051770989%2C%22xmax%22%3A-9222586.084776805%2C%22ymax%22%3A4854057.044223552%2C%22spatialReference%22%3A%7B%22latestWkid%22%3A3857%2C%22wkid%22%3A102100%7D%7D
geometryType: esriGeometryEnvelope
inSR: 102100
outFields: *
outSR: 102100
returnGeometry: true
spatialRel: esriSpatialRelIntersects
where: 1%3D1
2) I took a look at a response from of the requests that succeeded and it looks like there were 83 features.
Here's a request that failed - it ran for 103.9 seconds and it does not have a response body:
https://services.arcgis.com/eP8V7tJrYd6ZUBkc/arcgis/rest/services/OurFeatureLayerName/FeatureServer/...
Here are the response headers for this request - notice the "Miss from cloudfront" in the x-cache header:
access-control-allow-origin: *
cache-control: public, max-age=0, s-maxage=0
content-type: text/plain; charset=utf-8
date: Wed, 22 Nov 2017 20:46:25 GMT
etag: sd304_-148574809
last-modified: Wed, 22 Nov 2017 14:47:48 GMT
request-context: appId=cid-v1:f95092ba-7fdd-4452-a0e3-c1cfdc4250e2
server: Microsoft-IIS/8.5
via: 1.1 e9b460d4fbe79e1b34e06840ef460c69.cloudfront.net (CloudFront)
x-amz-cf-id: H-tXg_YB16glnrANqEGCKjE5SD-QUPaeFNVwAsbBHSD9QCIj6hPouA==
x-arcgis-instance: MTSDS_Web_IN_33
x-cache: Miss from cloudfront
x-powered-by: ASP.NET
Or I get a response with headers like this - noticed the "Error from cloudfront" in the x-cache header. This request ran for 180.05 seconds. Again, it does not have a response body.
accept-ranges: bytes
access-control-allow-methods: GET
access-control-allow-origin: *
access-control-max-age: 300
age: 23
content-length: 75
content-type: text/plain
date: Sun, 12 Nov 2017 20:06:46 GMT
etag: "e2651e71c06f4a6d095cb118ebfc79e2"
last-modified: Wed, 28 Jun 2017 00:39:39 GMT
server: AmazonS3
via: 1.1 e9b460d4fbe79e1b34e06840ef460c69.cloudfront.net (CloudFront)
x-amz-cf-id: xwGkF7ZwzhL61SL5AZWFKvI6fGFMghwBkyEpTII4GFKslWBXSSWwJg==
x-cache: Error from cloudfront