So I would suggest a compromise between your quick method and your reliable method. Flag pieces that can capture a piece without moving to its space, and use your reliable method on these while just checking if other pieces can move to the King's space.
I think I managed to even use some acceleration for the pieces that can perform 'locust capture', by avoiding you have to generate all their moves, and just limit it to moves that could potentially hit the King. This would be the move that already delivers check, and the moves that that mutate a square along the path in a way that would allow a slider leg of the locust capture to pass. Which is what was already done for direct captures too.
The issue was that the test for being already in check was done with the King taken off the board, by comparing the destination of capture-capable moves with the King square. But this was tested only on a final leg, and not for a non-final leg, where the capture would be a locust capture, and the piece would move on after it. I now compare the (temporary evacuated) locust square with the king position too, and if it matches make the move go over to the next leg to see if it can be completed. (For an Advancer that would always be possible, but a Long Leaper it might not be.) If the move can be completed, the locust square would be marked as attacked. This will then make the 'check' message appear.
But more importantly, squares on the second leg would also get marked as squares where a check could be discoved. This was a bug that did not yet express itself. A Checker diagonally adjacent to the enemy King would not deliver check if a friendly piece was immediately behind that King, blocking the landing square. But that blocking piece would then essentially be pinned, and its moves should not be highlighted. The accelerated test would only have noticed that if the second leg of the Checker capture would have added that Checker capture to the moves affected by mutation (= evacuation) of that landing square.
I think I managed to even use some acceleration for the pieces that can perform 'locust capture', by avoiding you have to generate all their moves, and just limit it to moves that could potentially hit the King. This would be the move that already delivers check, and the moves that that mutate a square along the path in a way that would allow a slider leg of the locust capture to pass. Which is what was already done for direct captures too.
The issue was that the test for being already in check was done with the King taken off the board, by comparing the destination of capture-capable moves with the King square. But this was tested only on a final leg, and not for a non-final leg, where the capture would be a locust capture, and the piece would move on after it. I now compare the (temporary evacuated) locust square with the king position too, and if it matches make the move go over to the next leg to see if it can be completed. (For an Advancer that would always be possible, but a Long Leaper it might not be.) If the move can be completed, the locust square would be marked as attacked. This will then make the 'check' message appear.
But more importantly, squares on the second leg would also get marked as squares where a check could be discoved. This was a bug that did not yet express itself. A Checker diagonally adjacent to the enemy King would not deliver check if a friendly piece was immediately behind that King, blocking the landing square. But that blocking piece would then essentially be pinned, and its moves should not be highlighted. The accelerated test would only have noticed that if the second leg of the Checker capture would have added that Checker capture to the moves affected by mutation (= evacuation) of that landing square.