It isn't the number of links in the TOC alone that determined load time. It's the structure of the book as well. What happens when you open the TOC is that the reader software calculates the page number of the destination of every link. If the book has too few page-breaks/too complex a block level structure, this operation takes too long.
This is just bad design on SONY's part, they should really be doing that calculation when a link is activated.
LRF files created by html2lrf are much more complex than those created by BookDesigner, since html2lrf supports a whole bunch of block level formatting that BookDesigner doesn't.
One technique that may speed up loading of the TOC is to increase the number of page-breaks. You can generate an LRS file with the --lrs option and count the number of Page elements to see how many page breaks the LRF file has.
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