theres no reason you cannot be strong, massive and fast. While you might not be the fastest as a little guy could be if you work on your cardio, sprinting, and agility using ladder drills, sprints, ect ect you will increase your reaction speed as well as your total speed. Consider incorperating weights into cardio sessions, barbell complexs are great for this also.
The most important way to increase speed and still be big is being flexible. Your speed is generally (stride length) x (strides/min) most of the time I see guys who look like their chugging it down field, but really aren't going anywhere this is because their legs aren't flexible enough resulting in shorter stride length. Yoga is great as well as just some static and dynamic stretching. Really work on your legs and back, but dont neglect stretching your arms and chest which do play an important role in speed.
What i'm trying to say basically is that dont throw away all your hardwork to get the body you have, try to adapt it to becoming faster. And slowly lean out and slim down to achieve your speed goals.
but if you have 18inch arms, and your body is in proportion to that, and your BF% is some what low, i think thats your biggest asset in RBSD, i dont know many people who'd try and f*ck with that. lol