+RTS -AL<size> controls the total size of large objects that can be
allocated before a GC is triggered.  Previously this was always just the
value of -A, and the limit mainly existed to prevent runaway allocation
in pathalogical programs that allocate a lot of large objects.  However,
since the limit is shared between all cores, on a large multicore the
default becomes more restrictive, and can end up triggering GC well
before it would normally have been.

Arguably a better default would be A*N, but this is probably excessive.
Adding a flag lets you choose, and I've left the default as it was.

See docs for usage.