Reloading a stage happens single threaded

[WD-stefaang]

Description of Issue

From https://groups.google.com/forum/#!topic/usd-interest/JCGGHoz9e2Q

I'm trying to understand why it's faster to open a stage than to reload it.
From the logs it seems like the opening a stage is multithreaded while reloading it happens in a single thread...
Loading (including fetch) in 3.5 seconds, reloading (no fetch) in 8.3...

https://pastebin.com/Xsjqva6p

Additional info on the stage: it's simply the kitchen set but with all payloads referencing to an S3 URI.
Note that this is on a high latency, low bandwidth environment, this is to make the issue more noticeable.
More info/questions here.

A) is it possible to make reload multithreaded like opening a stage?
B) where to put the slow request code to check for asset changes in a resolver: Resolve, GetModificationTimestamp or UpdateAssetInfo? Please document.

Steps to Reproduce

1. Add timer logging to the resolve, update assets and fetch calls of a resolver (e.g. the S3 resolver)
2. Open a stage referring to multiple assets, the indentation of the logs seems to indicate multiple threads.
3. Reload the stage and notice it takes much longer.

System Information (OS, Hardware)

Ubuntu

Package Versions

Basic USD 18.11 build + S3 resolver plugin

[jtran56]

Filed as internal issue #USD-4916.