Part of the issue with speed on the flats, or anywhere else is that of cadence also known as how fast you pedal. It turns out that rookies, such as myself when beginning cycling, tend to pedal very slowly. It has been shown by people who study these things, that increasing cadence will increase speed with less effort. If you were to watch some videos of pros, it can be seen they pedal at about 90 RPM.

That was the situation with myself also until I practiced pedaling. I would put the bike in a lower gear but maintain the same speed by pedaling faster. Then I would go to a still lower gear, say for several minutes at a time and do some repeats. After a while, going to my normal gear, the usual cadence seemed really slow. Eventually, with enough repeats, you end up going faster. Just like playing piano, the more you practice, the faster you can play.

I would add that under no circumstances can I be considered a fast cyclist. Good luck.