Its not the amount of gears that are important. However, 21 speed is a 7x3 set up, which is used by cheap mountain bikes. The 7 cog on the back is most likely a freehub, as opposed to a cassette. You may have an aluminium frame, but it will not be the lightest. Also, I would say that you have a 26" wheel. But the real deal breaker is going to be your tyres. Nobbly off road tyres, probably quite heavy, since the lighter race tyres are going to be expensive.
So, a heavy bike with a lot of rotational weight and rolling resistance. On a flat road, probably about 16 or 17 mph. However, it is not the bike but the rider. You may get more out of it, although that weight will be a bumber on hills. Another consideration is your gear ratio. Many rear cogs will have an 11 on the small gear, but the larger gear, that depends. Maybe 32 or more. However, the ratio on the front, especially on the big cog, are going to be telling. If you look at a track bike, it is a single speed with a massive cog up front. Slow to get rolling, but as long as you can power through it, you could get a lot of speed if you can maintain a decent cadence. Many mtb gears though are going to be smaller up front, as your bike is going to need smaller cogs to give better performance off road.
So although you may get a quick bike, it is not going to be as fast as something with big skinny wheels with over 100 psi in the tubes.