OK, coming from a total noob - and not having seen any such topic using the search function ...
I stumbled across this today (when looking about) on the procycle.us website:
"The stock TTR125 flywheel is extremely heavy and was designed to make the TTR easy to ride for beginners. If you're looking for better overall responsiveness this is your best "bang for your buck" performance improvement for the TTR.
A lightened flywheel greatly increases bottom-end acceleration, and improves throttle response. Allows you to flip the front end up easily in 1st, 2nd, or 3rd gear to hop over logs and obstacles while trail riding without having to gas the throttle and pull up on the handlebars!"
Hope you don't mind gear changing.... your foot will be tapping a tune on the gear lever...
engine flexability will go down the toilet.. be alot easyer to stall.. you will need more revs
to take off... be harder to start on the kick...