If I understand right, then when you comment the first (larger) bit out and uncomment the second bit it toggles at 0.7MHz?
The interrupt flag is not cleared so it will immediately jump back to the ISR. The second bit is not in an if statement so it will always be executed. So it will keep setting the pin in the interrupt_at_high_vector and clearing it in high_isr.
You need to make sure the flag is always cleared (if it is not cleared automatically)
What are you wanting it to do?
The mismatch condition will set the interrupt flag, and reading the port removes the mismatch, but doing so does not clear the flag, you must still clear it manually inside the isr.
From port B circuitry shown in the datasheet (Figure 3-4) you can tell that there is no way that WR (asserted during port writes) could remove the mismatch when the pin is configured as an input (TRIS bit is 1), because the WR signal does not affect anything but the "Data Latch", and the output of the data latch doesn't go anywhere because the tri-state buffer is in high impedance when the TRIS bit is 1.
The information missing here is that a port write also triggers port read, because all writes are really read-modify-write operations. The datasheet says this explicitly about port A, but I suspect it works the same way on port B. This means that when you write to the port, a read is actually performed as well, so RD is asserted, removing the mismatch.
However, the mismatch being removed when you write to the port does not affect the fact that the interrupt is called, because the flag is still set, which is the only thing you need to worry about. Your code does not change, you would still perform a read and then clear the flag when you enter the isr.
You may be wondering if a read or a read-modify-write operation occurs at exactly the same time as the input change may make it miss a mismatch condition. It does not, the flag is should still be set. This is because of the input latch controlled by a signal Q1 (which is not mentioned in the text). It basically keeps the input value for the mismatch comparison during the part of the read cycle in which the RD is being asserted, so that the mismatch is still caught.
Best Answer
You would check the state of the PORTB pins you are interested. You can also store their status on a global variable for comparison on the next time around, this would allow you to determine which pins have changed. Here is a sample of what your CN ISR might look like: