I'm not sure what you mean by "more troublesome". The whole point of this plugins integration is to manage relationships between books on a shelf in Goodreads versus those you have in Calibre. If you don't intend to put the book on a shelf of some kind, then there is no real point in linking it with the plugin
However it sounds like you do intend to have them on a shelf, for the ones that are not already on one. In terms of the "slow and unresponsive" comment, I can only assume that you have a very large number of books on that particular shelf you are trying to add them to. In which case yes that will be slow to launch the dialog. What sort of numbers are on the shelf that you find slow to add to? Not that I can do anything about it to speed it up, other than to remove the code which downloads all your shelf contents each time so as to identify which are new books that need adding. However that would be "robbing Peter to pay Paul", because of course unless you were careful to only use this function to add books that did not exist on the shelf you will end up doing more calls.
However, if your starting point is that you have some books already setup on Goodreads on a particular shelf, is there any reason why you chose not to use the "Sync from shelf" functionality, to make sure you had Calibre records for all the books on that shelf that were of interest? You can create empty book records if they are books you do not have as yet and link to books that you do have in Calibre all in the one step?
It did occur to me btw when I originally write the link feature in the menu (which was initially just for my own test purposes until others said they found it useful) that people might want a dialog for multiple selection. However as they already have similar functionality on the Add to Shelf and Sync from Shelf dialogs, it seemed a bit overkill. Obviously if it is something that more people would like to see rather than using those other functions then at some point I can take a look at adding it.