This is why you're becoming slower...
It shrinks the part of your brain that stores your memories.
Staying inactive for long could eventually lead to atrophy of the medial temporal lobe which is where memories are stored.
Inactivity leads to thinning of the lobe and physical activity isn't enough to reduce the effects.
However, performing cognitive tasks may increase your chances of maintaining your temporal lobe as opposed to mentally inactive sitting.
Standing while working also doesn't work as it is just substituting one inactivity for another.
Getting consistent low-impact exercise can be helpful more than intense workouts after a day of working. As high intensity workouts after a day of inactivity can increase chances of cardiac arrest.
Take short walks after every three to four hours at work.
Take the stairs instead of the elevator.