Difficult ignition 2017 Mustang GT

We have a 5.0 and for a couple years it has off and on difficult ignitions. Almost like it the "strengths of the ignition isn't full--takes about twice as long to start as it should. You can hear like a struggling pitch until it gets to full power and starts. It's definitely not the battery as that was replaced last week and just yesterday the symptom returned. Seems to happen most (not always) when car has sat for a long time in the heat.

Any ideas?