If you let the eDiscovery Export Tool time out, it will eventually give you a link:
which has the registry edit below. No explanation is given, but it does say that adjustment of the number (in some direction?) can help.
Windows Registry Editor Version 5.00 [HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Exchange\Client\eDiscovery\ExportTool] "DownloadConcurrency"="2"