D memory consumption/runtime speed problem

[bearophile]

sybrandy:
> private immutable uint ONE_BYTE_VAL = (1 << 6) - 1;
> private immutable uint TWO_BYTE_VAL = (1 << 14) - 1;

Use "private const" in D1 and "private enum" in D2, there's no need for an immutable here.
In your code there are now no useless memory allocations, so the exit(0) trick is not needed.

Bye,
bearophile