What made a submarine firing solution difficult?

How long did a WWII submarine need to compute a firing solution and launch torpedoes?

There was no standard duration: it depended on target movement, observation time, submarine positioning, and complications such as zigzagging.

There was no single fixed time for a Second World War torpedo firing solution. A submarine had to expose its periscope briefly, estimate the target's bearing, speed, and course, then lower the instrument before it was spotted. The crew repeatedly updated that estimate while manoeuvring the boat into the correct firing position. Human observers, calculators, and communications remained inside the loop rather than producing an instantaneous solution. Zigzagging compounded both elapsed time and error by changing the target's future position irregularly.