Microwave kiln & microwave
My experience is with a large Hot Pot microwave kiln and an old 650W microwave oven at full power. I did not count the number of firings but I guess between 200 and 300 – definitely more than you have experienced. Mine is also taking longer to do its fusing and is geting worse. I also vaguely recall reading somewhere on the Internet that microwave kilns “wear out”. From this I assume our micowave kilns have reached the end of their useful lives.
I suspect the dark coating inside the microwave kiln (this bit that absorbs microwave energy and re-emits as thermal energy) deteriorates (oxidises?) over time so becomes less effective. I believe it is possible to re-coat microwave kilns but I have not tried it.
Mine has developed thermal expansion/contraction cracks so now heats up unevenly. Again a re-coating might resolve this at least temporarily.
Firing times were usually about 8-10 minutes, but now are taking longer and longer (currently about 40% longer). I couldn’t fire any faster with my 650W microwave oven but comments I’ve read on the Internet suggest that higher wattages can cause thermal shock problems with the glass.