You would say, "I will graduate [later]."
Since you have taken a year off, your graduation will be pushed back (e.g. from 2018 to 2019 (later - further in the future)).
"slow" has the meaning of speed or how fast things can done, but "later" has a meaning of a time after a time you have mentioned.
"will graduate" is needed as you are using simple future tense to describe something that will take place in the future.
" I have taken a week off. I will complete my project slowly."
(focus is on the fact that you are taking your time to do it though there is still a meaning of a future completion with "will complete")
"I have taken a week off. I will complete my project later."
(focus is on the fact that you will complete it at a time further in the future, without any mention of speed.)