How long did Atlantic convoys actually take?

Did wartime Atlantic convoys take 10 to 13 weeks or travel at only four knots?

No. The named examples place slow crossings around two to three weeks; four knots may describe net progress in conditions, not the convoy's intended speed.

Ordinary Atlantic crossings were closer to roughly two to three weeks, even for slow convoy series, rather than several months. An HX example crossed in about fifteen days and an SC convoy took roughly three weeks despite suffering a serious attack. Zigzagging, currents, weather, and assembly can make a convoy's net progress look slower than its ordered speed. A Type VII U-boat's surface speed further undercuts the claim that only four-knot convoys could be pursued.