I recently bought a stock 2021 Mazda 3 non-turbo 2.5L AWD automatic. The thing that bugs me though is the initial throttle lag. I've seen ads for the throttle response controllers, but will those eliminate the lag? Or is there other fixes to get rid of it?

 

You will likely want to contact a reputable tuner about this. Lots can affect this. But from what I know from other tuning platforms is electronic throttle can sometimes be altered in the tables to change the ratio of your pedal to the electronic throttle body. There is also things like throttle percentage to shift points based on rpm and/or vehicle speed. I wouldn't do this yourself.