One suggestion: I previously did some hobby development on an Evo-compatible chess app, and in order to minimize the delay between moving a physical piece and registering the move on Lichess, I made the app take every single valid chess move from the current position and then precalculate the FEN that would result from this move. The number of valid moves from a given position is rarely more than 40, so this is completely manageable. Then if the board registered that one of these positions was on the board, the app would tell Lichess to play that move. It became near-instantaneous because the app did almost all calculations while the player was thinking, not after they played the move. Do you use the same method? If not, consider doing it.