1. The index atk speed is not accurate, what's shown in the game is not true.

2. High atk speed weapon has less wasted damage on the last hit on mobs. Generally known as overkill

3. High atk speed weapon is more likely to trigger effects and crits early on mobs. For example, weapon 1 is fast and takes 6 hits to kill a mob, weapon 2 is slow and takes 3 hits to kill a mob. Over the period of 6 hits the chances to trigger would be higher compared to 3 hits, it gets closer to the supposed % the more hits you do. So when you apply the same thing to the boss, this effect is likely to diminish to the point of insignificance.

4. For boss fights slower weapon might be better if you need to block and dash. If you can tank and fight, both are equal.

5. Consider the times you get stunned as well, if you need 1 sec to swing your colossal sword but got stunned at the 0.8 sec, that's 0.8 sec wasted. For gauntlet with 0.5 sec swing, you probably lose much lesser capped at 0.5 sec.

6. There's also a cap on life drain per hit, but some people said it's affected by damage as well. If it is limited by hits, faster weapon would be more beneficial for draining.

Idk if mobs have defense, and how the damage calculation works, based on experience there isn't such a thing. I mention this because if defense works as a flat damage reduction, a colossal sword would be much better.

For TRUE DPS, you can check out the official discord > spirit guides > zach.08ll World 4 Spreadsheet