You can essentially get any result you want from the "3D Mark Wild Extreme Stress Test" benchmarking utility.
On the same iPhone 15 Pro Max, for example, I collected the following three very different results:
Simply running the test with my phone as I found it (on battery):
- Best loop score: 3590
- Lowest loop score: 2255
- Stability: 62.8%
Immediately after rebooting and in airplane mode (on battery):
- Best loop score: 3554
- Lowest loop score: 2768
- Stability: 77.9%
Continuing in airplane mode and not rebooting (while charging):
- Best loop score: 4114
- Lowest loop score: 2423
- Stability: 59.9%
Note that I would recommend against making any assumptions from the above. Much greater rigor would need to be employed to have accurate results worth using.
To really be able to compare results from the iPhone 13 Pro Max and iPhone 15 Pro Max, you would need to make every effort to reduce as much as possible any variability in conditions between the tests on each device.
Personally, my opinion of the iPhone 15 Pro Max is that it is the very best iPhone to-date based on the performance, temperatures, construction, materials, and form factor.