• try add. leverages a custom path since the internal implementation combines existence check and the insert. so avoids one of the two lookups. and eventually less code when we burn only target runtimes
• use the items constructor. less code and leverages an internal AddRange...

// It is likely that the passed-in dictionary is Dictionary<TKey,TValue>. When this is the case,
// avoid the enumerator allocation and overhead by looping through the entries array directly.
// We only do this when dictionary is Dictionary<TKey,TValue> and not a subclass, to maintain
// back-compat with subclasses that may have overridden the enumerator behavior.

it effectively does an array copy of the internal dictionary entries. this should speed up the hot path since the copy is done in Emit(LogEvent)

also avoids enumeration and the value lookup in the source

foreach (var key in _properties.Keys)
            properties.Add(key, _properties[key]);