When I drive on Highway speed the RPM around 1800-2000 so i think the transmission OK
or i worng ?

it'* not something radicaly , my last buick reduce the speed like 1 km\h in 1 second and that Buick reduce it in 3 km\h in 1 second .
I open the throttle body and clean it , it was dirty . but it'* not solve the problem

if the other ones you are use to older buicks its not the same, the newer ones do slow down faster. all gms after 2000 or so do it, sort of help the brakes i guess. make sure your air pressures are up and the rotors dont look hot. but a slight difference seems normal, you get use to leaving your foot on the throttle a little if you want to coast. the ones after 05 or so when they went to drive by wire are worse, they change the distance you have to leave it, so you have to leave it more on.

The last buick was the same , 2001 lesabre .
in the past i had 2004 lesabre .
There is something else that I can open and clean it ?

you check and see what the noise was? if you have a exhaust leak before the upstream O2 it wont run properly