Hot Paths Cleanup [API-2059]

### Argument Checks

Many of our methods look like:
```csharp
public void DoSomething(Thing thing)
{
if (thing == null) throw new ArgumentNullException(nameof(thing));
```
where we *know* that we are (in theory at least) never going to pass `null`. This means we are adding a fair amount of checks in the "hot paths" and is this a performance issue? It may be dangerous to just remove all the checks, OTOH using the new `#nullable` feature of C#, we may do:
```csharp
public void DoSomething([NotNull] Thing thing)
{
// assume it's not null
```

### CancellationToken

Many of our methods accept a `CancellationToken` even though our public API does not expose it, and therefore it's always going to be `CancellationToken.None`. Can this be a performance issue in hot paths, and should we just remove the parameter since it's never used?