By default Bob saves media files on the server and creates a tiddler with an external link to the media file, so when you open the sidebar it has to load all of the images from the server, which is probably where the delay is coming from.
The files urls are all relative so they don't work when exporting to a single file wiki because the url isn't the same.
If you want to make this not happen then turning off the file server will make the standard behavior of embedding media files in the wiki as base64 encoded tiddlers.
It makes the wikis larger because they contain all the media, but everything is loaded when the wiki page loads so you don't get the same delay, and exporting as a single file wiki keeps all the media accessible in the html file.
Eventually I plan to make exporting single file wikis keep the media usable, either embedding the base64 encoded versions in the exported file or exporting a files folder that can be placed next to the html file to make the media accessible.