Not sure what you are asking.
Rendering time relates to the number of calculated pixels with filters. The complexity of the image you apply on the filter doesn't really add up.
Filter is stored in the def section, objects use them just as they were custom markers/patterns/lpe-s, the way of "linking" is the same.
The rendering time/resource need is different, lpe-s are vectors, all of the nodes are processed when you select a path with an lpe.
Even if you see only a small part of it on screen but select it, you would need to wait for inkscape to respond on a complex lpe.
Like a sketch lpe stacked on a hatches lpe, with a pattern along path on top for example...
Patterns may need less resources though.
Then, if you have a complex filter, with various blurring and convolution matrix, rendering time can be cut short if you zoom out.
Exporting at a decent resolution is what can take huge resources.
(The latter filter primitive is much affected by the zoom level. If you want to reproduce the same look on different zoom levels of the same object, it doesn't work.)
By the way, good idea on a bubble filter.