There's definitely something not right with the update speed.
I don't have the answer either, but I did mine last month and it took three tries, and all evening, to get it done.
I was very worried that when it crashed, It would corrupt the memory card, but thankfully it restarted and progressed OK. It just took forever.
Having said that, my little old 4.3" TomTom has lifetime updates, and it can take a few hours to do it as well.
I update it every six months, even though it doesn't get used much. It's free, so why not eh.