Disable update-in-place.  In its current form, it has a serious bug:
if the thunk being updated happens to have turned into a BLACKHOLE_BQ,
then the mutable list will be corrupted by the update.

Disabling update-in-place has some performance implications: many
programs are not affected, but one program in nofib (nucleic2) goes
about 20% slower.  However, I can get it to go 300% faster by adding a
few strictness annotations and compiling with -funbox-strict-fields.