I wonder if selectively disabling D's collector would help? Turn it off during the allocation phase to prevent thrashing. The runtime would be forced to allocate from the OS rather than try to collect. It could then be enabled and run when there was known to be garbage to collect.

Another option would be fall back to pre-allocated object pools rather than naively calling new. I know it is a fairly common optimization in Java\C# when the GC is causing performance issues.