Why would my car have a slow start as if the battery was dying then it starts like normal battery alternator and starter tested all good

Hello, one possible cause is that the battery is somewhat compromised, and as such the engine labors to start on the first, cold engine start. With the engine sitting overnight, all the oil drains to the oil pan, all the engine parts cool off. This requires more power and energy when attempting to crank over the engine in comparison to when the engine has been run already and all parts are well lubricated and warm. I would suggest having the battery tested with a good quality tester to determine what the state of charge is, if there is any internal bad cell, and how well it can handle a load. I do understand in your question that you had the battery tested already, but I do wonder what kind of tester may have been used. Some "borderline" batteries can test "good" when in fact sometimes they are weak at best. The best battery testing equipment can put an active load on the battery and measure how quickly the voltage drops when under this load. A good battery should have a state of charge of above 12.5 volts, and should stay somewhere above 10 volts and hold on a load for 10 seconds or more. A battery that fades and continues to drop below 10 volts under a load is one that is definitely in question. As a second thought, there is also a chance you have a very small "parasitic draw" on the battery, meaning something in the vehicle is staying energized after you leave the vehicle. Similar to having a light staying illuminated, some electronic components stay active for a short period of time after shutting off the car, then go 'to sleep' shortly thereafter. A relay that is stuck active could slowly drain a battery overnight. Usually this would result in a dead battery though, not one that just cranks the engine over slowly. In any case, I wish you good luck!