WAP-7 - तेरे जैसा यार कहाँ , कहाँ ऐसा याराना , याद करेगी दुनियां तेरा मेरा अफसाना. - Arjun Rai

There is no 'speed saturation' per se. One can design a locomotive either for heavy haul duties or high speed.
For a given power rating, a heavier loco will be able to heavy haul duties while a lighter loco will be better at high speed duties.
With both rated at almost 6000HP, the WAP7 and WAP5 are the best examples for this. The WAP7, heavier of the two at about tons, has a very good starting TE and can haul heavy rake but has poor acceleration beyond 50-60 kmph and practically no...
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hauling capacity beyond 140kmph. The WAP5, a very light loco at 78 tons, has relatively less starting TE and is not particularly great at low speeds, but beyond 60 kmph, starts to show much better performance than the WAP7.
The WAP7 can be made even faster, by reducing the loco's weight and playing with the gear ratio, but that will come at the cost of reduced hauling capacity.
There are many other parameters which make a WAP7 style design a poor choice for high speed operations.