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

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