This is a very interesting question.
Usually we talk about the power of the microwave is the output power, while the microwave also has an input power.
Since the exact model not listed in this case, so let's use a similar model
High Pointe's EM925AWW-B as an example. According the
deiscription:
Microwave Input Power Rating: 1500W
Microwave Output Power Rating: 1000W
According to what
@azdp checked, the conversion efficiency of the APS2200 PSW inverter is 90%.
So it needs at least 1666W at the battery side, which means at least 138A current is needed.
The maximum continuous output current of Chins' battery is 100A, so it can't provide the enough power to the microwave.
Although the maximum peak current is 300A, it can only make the microwave start, but not to make it work properly.
In the case of insufficient input power, it makes sense that the heating time of the sandwich becomes longer.
It does seem that two batteries are needed to solve this problem. Can temporarily add a battery to verify.
If the above explanation holds true, the interesting part here is that when the input power of the microwave is about 3/4 of the rated power (1280W->inverter->1152W), the heating sandwich time is actually extended by 10 times (2min->20min). Wow! Or does it just use multiple start-peak-moments to complete the heating?