The bike in question:
https://archive.trekbikes.com/us/en/...7_1_fx/details ?
You could save a pound by swapping the fork and you would still have a 26-28-pound bike with w low-level drive train and heavy wheels, designed for casual, comfortable low-speed riding around town..
You could save another pound by spending another $300-$400 and upgrading the drive train a few levels.
After all that money, I honestly doubt you would feel any difference in the bike's performance.
As @
Rolla said, you could do a lot more to change the bike's responsiveness by investing $400-$500 in lighter wheels and tires.
After all that, you would end up with a bike almost as good as a new bike which would cost the same.
I would not put a penny into that Trek. I would ride it it and enjoy it as is, while saving up for a better bike more suited to the type of riding I actually do. If you actually do casual, low-speed riding around town, the bike is already perfect.