`parseArchive of GZIP-ed tar file: ZlibException (-3)` is slow

[hseg]

In downloading extra-deps for pandoc d6dbbb89, between two github deps there
is a long pause (on the order of 5-10 minutes per dep) after the line
[debug] parseArchive of GZIP-ed tar file: ZlibException (-3).
Either more work is being done behind the scenes than is being logged, or the
exception causes stack to stall unreasonably. Either way, it dominates the
fetch time for these dependencies:

• texmath:
  • 3429ms to clone+reset+update submods
  • 95ms to archive
  • 1096564ms for the exception
• citeproc:
  • 1558ms to clone+reset+update submods
  • 46ms to archive
  • 289220ms for the exception