The cold start system isn't working and the engine's control system
can't offset the deficiency during the first few minutes - thus the
erratic performance for the first few minutes.
Fix the cold start and the other will be resolved as well.
Using regular fuel in a 10:1 compression engine, especially in hot
weather is a bit self defeating because the engine's knock sensors
instruct the computer to retard the ignition which reduces power.
Forget the Km / L reading; compare Km/ $. I suspect there will be some,
but not too much difference, between regular and premium.
And about being cheap with respect to these cars - don't, there are no
shortcuts, cheaping out comes back to bite YOU, nobody else.