-H alone causes the RTS to use a larger nursery, but without exceeding
the amount of memory that the application is already using.  It trades
off GC time against locality: the default setting is to use a
fixed-size 512k nursery, but this is sometimes worse than using a very
large nursery despite the worse locality.

Not all programs get faster, but some programs that use large heaps do
much better with -H.  e.g. this helps a lot with #3061 (binary-trees),
though not as much as specifying -H<large>.  Typically using -H<large>
is better than plain -H, because the runtime doesn't know ahead of
time how much memory you want to use.

Should -H be on by default?  I'm not sure, it makes some programs go
slower, but others go faster.