There are a few different possibilities. Before getting into particulars, you should probably start by asking yourself:
"What else was changed when the camera began starting up slower?"
That will almost always point you in the right direction to what is causing the different behavior by the camera.
- Did you use a different battery than before? Particularly a third party battery?
- Even older genuine Canon batteries in newer model camera bodies can sometimes not work as smoothly as expected.¹
- A different memory card?
- Have files not created by the camera been added to the memory card?
- Has the memory card been formatted in a device other than the camera?
- Has the memory card been most recently used in another camera?
I've experienced a similar delay when using older third party batteries with newer camera models. Sometimes, after the short delay, the camera will recognize the battery as "genuine", sometimes not.
This is due to the way third party batteries are "reverse engineered" to work with models that are in use when the batteries are designed. Canon will occasionally tweak their battery communication protocol that makes the third party batteries less than fully functional with the new models. It usually only takes a few weeks for the third party battery makers to update the firmware in the batteries they are making to comply with the changes.
Since the camera accesses the memory card during startup, anything to do with the memory card that throws the camera a "curveball" can delay successful completion of the startup routine.
¹ For example, when Canon released the 5D Mark III in 2012 and the subsequent 7D mark II in 2014, they started calling on code in their battery communication protocol that had previously been "hidden" (i.e. present in the code but not used in actual camera/battery communications so that third party "reverse engineering would not detect it). This made almost every existing third party LP-E6 clone at least partially "obsolete" when used with one of the newer cameras. Unfortunately for Canon, they also accidentally rendered some older genuine Canon LP-E6 batteries equally non-fully functional. The updated chargers supplied with the new cameras would refuse to charge them as "not authentic".