when you open the wallet, side by side with the staking button there's a precomputing button, just click the grey area and it'll become blue.
enabled!
enabled!
Ah. Thank you very much maybe this will bring me some luck in the staking.
Guys, when I download a new update, all synchronized data will remain? Because after a few days I am still on 30% due to my slow internet connection. 
Yes, all the synchronized data should remain intact. I would still suggest you download the chain snapshots because they are much faster to download.
Not only the recent snapshot but also they need to download the new version of the wallet and it will surely increase block maturity speed and in future, we don't see any synchronisation problem any more.