Grohe LadyLux3 faucet flowing at 40% expected rate, even after replacing cartridge
If it's equal between hot and cold, it's at or after the cartridge. Since you've ruled out the flow restrictor/aerator, the easiest next thing to take apart is the quick connect leading from the mixing cartridge down to the flexible hose loop. It's K in this diagram and usually made out of plastic that might need some delicate handling to detach without breaking.



I wonder if you don't have some debris in there.

If you don't want to do try to detach the quick connect, you can also try removing the cartridge again and blowing into the outlet to dislodge the debris and shoot it back up to where the cartridge is.

If there's no debris, or even if there is, point the output from the hose to a bucket and turn on the water and the faucet. The pressure at that point, without the hose, will help you narrow down the problem. If you still get low pressure there, that would be very weird since the blockage can't be upstream of the cartridge.