Since a few months , backing up takes an eternity.
Size is about 10Gb , and uploading to Nabu Casa is so slooooowwww that I’ve had to disable it.
Your backup is 10Gb?! Not trolling… legitimately asking… what is in that backup?
I seem to recall that Nabu Casa backups are limited to 5Gb. If that’s still the case, then you might not have any working backups at all
Well it’s 10gb estimated but actually just 3.3 gb.
And I have also backups on my NAS.
Still I updated Core yesterday at around 9pm and it finished at 00:45 , so just a little less than 4h …
A Core update taking 4h is pretty much unheard of. What hardware are you running HA on & how is it connected to the network?
Yeah, send a ticket to support, as if your system can get it onto the NAS quick your system probably isn’t the bottleneck.
But before you send in that ticket, double check up ISPs upload speed and do a speed test that checks the upload speed (if you’re in the 3Mbit or lower territory, that might be your problem).
My backups are around 40GB 
But that is mainly because I store a few camera snapshots for notifications and I have my main instance of BirdNET-Go as an app. And my database alone is around 14GB.
I’ll test a few more things and do a support ticket.
Upload speed is ok (around 20mb/s)