Car has delayed acceleration after stopping, and when the AC is on? No lights are on?

I have a 2006 Mitsubishi Galant Sedan,
and had quite a bit of work done on it but
still have not found a solution to its
problem. I took my car to a mechanic who
I thought was good, but only ended up
having to spend $2k on repairs for nothing.
Out of nowhere, my car started having
quite the delay in acceleration after coming
to a stop at stop lights and stop signs. This
is the case even if I try to floor the gas
pedal. The car appears to struggle even
more with this issue if the AC is on, yet no
indicator lights pop up, so I can't run
diagnostic tests on the car either, which is
obviously not making my life easier. The
car has had the catalytic converter
changed, serpentine and timing belt
replaced, and I believe an O2 sensor may
have been worked on, but it's been quite a
while since the repair so I could be wrong
about the sensor. Something to do with air
intake was possibly also done already, but
literally nothing has helped. I'm lost and
highly disappointed, because this car has
been sitting in my driveway as I have given
up on it, and it just hit 100,000 miles. Does
anyone have any idea what is wrong? I've
been scouring the internet for ages to no
avail. HELP ME PLEASE :(