could not go over 20mph

Hello everyone, came to this forum to seek for help.
I've an auto transmission chevrolet aveo 1.5.
Previously the car could not go over 40mph on the road so I took it to the service center and they said it was a problem from faulty gearbox.
Changed the gearbox and its now getting worse at 20mph on the road only.
Does not show any orange light at the dashboard.
Anyone encounter this before ?

I had same problem. Turned out catalytic converter honeycomb broke apart and got stuck in exhaust pipe. Removed cat and replaced with new. Cleared broken crap from pipe.