Well i faced the same problem for over a month, tried everything for diagnostics, the computer gave no hint DTC was always clean. Check engine light would come sometimes but again go off upon restart. The engine itself was dull, hitting the paddle hard gave sluggish acceleration but soft paddle seemed ok, that would leave me behind every other car on as soon as signal went green. The engine seemed to be choking, rev up would not convert into transmission it looked like transmission ptoblem. I checked following things.
Air filter changed monthly
Spark plugs changed 4 months ago
Throttle body cleaned 4 months ago
O2 sensors both cleaned 1 month ago
Exhaust system removed today
Oil sensor cleaned 1 month ago
Fuel pump removed (i was about to change my fuel pump but it worked fine on the bench after removal) today
Transmission oil checked (went through vitz owners manual it said transmission oil need not to be changed for lifetime) today
Then we found out the real trouble...
My car was once fitted with CNG but then the kit was removed. The sensor that deals with CNG is inside throttle body on left side of air filter it had gone bad and gave poor acceleration as well as poor mileage. Replaced the sensor and the car started running like a horse. Cost me only Rs 2500. You can also try step by step diagnostics as i did but i have to appreciate my mechanic he seemed to be a phd on efi engines, had a complete grip of the entire subject. He could have ripped me by changing everything possible but he was honest on every step. Hope you also find a good one. Plus many people say its the Catalytic that causes the issue its rubbish. We checked the engine with exhaust removed and it worked as bad as before. So exhaust system and catalytic was just fine even after 12 years running 02 sensors fine even after 12 years. Transmission oil fine after 12 years. Cars are made for running, there was only one sensor bad that too because of stupid CNG kit long time ago.
Hope it help you resolving your car issue as every case is different with every car.