Yes, functions with optional and named parameters have an outer function which basically does this:

if (offset != null) {
  offset = defaultValueForOffset;
}

callRealFunction(vertexBuffers, offset);

I believe (but haven't checked) that named parameters are even more expensive, requiring a map lookup.

if (!optionalParametersMapHas['offset']) {
  offset = defaultValueForOffset;
}

callRealFunction(vertexBuffers, offset).