I am guessing the exhaust is restricted. I had this problem when I first bought a performance exhaust for my Gilera Runner 50.
There is a little cone/cylinder at the top of the exhaust, in the manifold where it goes into the bike. What you have to do is get a screwdriver (one you don't need because it will get ruined), and use the screwdriver as a chisel and hammer away at the cone until it separates from the two welds holding it there.
Then you file away at the two welds so there is room to pull the cone out.
If you don't feel confident doing it, a motorcycle mechanic will most likely do it for you.
Once I removed the cone from mine, it added 20mph to my ped, but
Hope this helps.with yours being a 125, it will most likely add even more to your top end and acceleration.
Please bare in mind that this information is dependant on if your bike is a 2 stroke. If it is a 2 stroke, this will work and add performance to your bike.
However, if it is a 4 stroke, it will make no difference what so ever.