A couple things come to mind...
1) the line you are using now is too big. Way too big. On a 12m boat the deck hardware
is highly unlikely to be specced to handle 14mm line, so it am guessing you are having massive friction problems in your running rigging
. Stepping down to a smaller line is absolutely the way to go, even if you keep with double braid line.
2) 10mm spectra is still massively oversized. It probably falls within the deck
hardwares spec size, but you don't need that much strength. Assuming what you have is 12mm, the spectra would be roughly 2.5 times the strength at 10mm.
3) what I did (38' crusing boat) was to go with 1/4 (6mm) spectra, and splice on a cover where needed. This actually works out to be cheaper than a double braided line and it's is much easier to use, lighter, less friction, ect.
Alternatively you can go with a tapered endurabraid halyard
which will work out about the same price
as double braid poly, and give you most of the advantages. But I like splicing work so the extra fun with ropes doesn't bother me at all.
The one thing I always tell people is to look at the specs on your deck hardware
and make sure not to use anything above the maximum size. And keep in mind the MBL of the gear
. Winches on your boat probably explode between 1000 and 2000 lbs, so speccing line with a MBL of 16,000 lbs is a little silly.