FileVault irregular time remaining

I'm using a Macbook Pro Mid 2010 with Yosemite. I have a 256GB SSD installed. FileVault at first told me estimated 4 hours. I left my laptop on all night, and now it says 18 hours remaining... sometimes jump up to 20 hours, even over a day, pretty much stuck there... I cannot turn FileVault off until encryption is finished. I expected more from Apple product. Now I cannot use my laptop for another day or so. Let me know if there's a way to cancel this thing if it doesn't go through.