No, the page height will define how much data is sent over the wire to the extension.
However it is very likely that your extension ends up behind something else in the paint cycle and has to wait on its turn to re-paint itself.
Try creating a totally empty sheet and place the extension on it and do some selections to see if the problem still exists.
However if you have a expression exposed to your extension that aggregates tons of data on the server then yes it might need some compute time on the server but the data that will be transferred from QVS to your extension is still defined by the PageHeight property.
Not really, however most of the objects in the AJAX client renders very quickly.
Most charts are actually images that gets generated on the server and then sent over to the client so that is fairly fast.
Tables dont render all the content up front but page in the data as you scroll through them.
Pivot Tables however are quite intensive however.
However it is always a balance, I have come across apps that had over 500 objects on a single sheet (usually someone that reinvented the tabrow with text objects) and that will obviously slow things down.