/effect works as following:
/effect <player> <effect> [seconds] [amplifier] [hideParticles]

In the latest snapshot, the 'seconds' parameter isn't handled as seconds, but as game-ticks.

How to reproduce:

/effect @p minecraft:speed 100 1 true

What I expected to happen:

It would give me Swiftness II for 100 seconds.

What actually happened:

It gave me Swiftness II for 5 seconds.

Why? In 1 second, there are 20 game-ticks. 100/20 = 5 seconds, meaning that it handles the 'second' parameter as game-ticks, and not as seconds.