For a document of 400 pages there can be over 30 seconds of unresponsiveness before you can interact with the viewer. Is there any way to improve this behavior?
WebViewer has two main ways of loading xod files, one is by making HTTP range requests to load only the necessary parts of the file on the fly and the other is by downloading the entire file, processing the parts as they come in. This second way will be slow with large files because it's downloading and parsing the entire file. The first way will be much faster since it can request parts as it needs them.
Possible reasons why your server is using the slower loading method:
1) Your server isn't supporting HTTP range requests for the document and so WebViewer is falling back to the slower loading method. For better performance you'll want to support range requests.
2) You have set the WebViewer option "streaming" to true. This streaming option refers to streaming of the XOD conversion to the viewer and is only recommended for use in this case. For small documents it won't be too slow but for large documents we highly recommend converting the documents ahead of time. If a document is converted ahead of time you should set the streaming option to false (or don't specify it since the default is false).