It provides a 50% increase.
I have fixed the information displaying as 25% this morning (hopefully live next week).
This thread was added on March 31, 2020, with posts from JagexRyan.
Original Post
https://i.redd.it/t7947tbpwvp41.jpg
So it was planned originally to be 25% but then got buffed later to 50%?
Indeed! A LOT of stuff changes in pre-release.