The change that happened in 2019.2.8 was a switch to using single NSURLSession for all downloads. The reason for change was to reduce memory usage, as separate session per request meant not only more memory used, but there were also issues related to how iOS handles secure connections under the hood, keeping some resources for a while in use even after request is complete, resulting in memory usage increase with each request in a memory-leak like fashion.
Also, on mobile platforms network speed usually plays a more significant role, speed tests using great wi-fi connection are a bit synthetic.
We believe that lower footprint for handling requests is more valuable here and it's a worthy tradeoff to have slightly slower individual downloads.