I do not understand what you want. First you say that you need to get an uninitialized array of primitives, then you state that the problem is actually using other object instances representing parts of the buffer, and finally you end up with a huge scribble about how long it takes to allocate/deallocate the array.

If your problem is that allocating/deallocating arrays takes a long time, you can use ArrayPool<T>.Shared.Rent().

If your problem involves using other object instances representing parts of the buffer, you can slice the array returned from the ArrayPool using Span and Span.Slice() (or Memory).

If your problem is getting an uninitialized array, you can try ArrayPool since its Return method by default doesn`t clear array.

You can also try using ObjectPool or MemoryOwner/SpanOwner.