Higher speed on freighters is only usefull to decrease the time it takes before it reaches it's target when you're trying to establish the trade route. I stock my freighters up with Sensors, that way while they're doing their little shuffle between planets they're also illuminating parts of the map.

Once the trade route is established the ship icon only moves at 1 / turn. This doesn't mean the value of the trade route is diminished, it's just a game mechanic. The trade route generates X per turn based upon the distance and economic values of both planets. The icon moving slowly along the screen is there only to allow military to intercept and destroy trade routes. It was a simple way to put in 'economic disturbances' to shipping without having to bog down the game in trying to represent blockades.

So your saying that if I increase the speed of my freighters during trade, instead of the speed increasing on the map for my freighters, the income will so to compensate the factor of higher speed = higher income ?

No. Speed has absolutely ZERO effect on the trade route once it has been established. A 20 speed freighter and a 1 speed freighter will generate exactly the same income if you build them on the same planet and send them to the same planet.

Live and learn.