Automatic gear changes are very slow with a delay at the expected point where you'd expect a gear change. It can feel like there's a lag between putting your foot on the accelerator and the gearbox shifting gear.
When you put your foot down quickly on the accelerator the 'kickdown' function does not work and the car either stays in the current gear or goes into limp mode.
When you put your foot on the accelerator, or take your foot off the pedal, the car doesn't change gear as you'd expect. It can seem as though the car is struggling to select the next gear.
When changing gear the gearbox makes a loud grinding noise. This will be a more common occurrence on cars with manual gearboxes but can also happen with automatics. There can be various reasons for it but it always presents as a loud, unpleasant sound of mechanical scraping/grinding.