Weapon cooldown charges very quickly, charging to half-full attack cooldown in the first few frames. After it charges to full after those couple of tenths of a second, weapon continues charging as normal.

Steps to Reproduce:
1. Get any tool/weapon with non-default attack charging time, i.e. an axe in slot 1
2. Have your selected inventory slot to not be the tool/weapon, i.e. slot 2
3. Scroll/press hotkey to go to the slot with your tool/weapon, i.e. slot 2 to slot 1
4. Attacking during the split second will deal more damage than minimum charge time intends to (~2% damage for axes). Attack strength is about 30 to 50% of charge meter using 60hz monitor (up to 40% damage for axes, which is 20x more damage than intended).

If you replaced step 2 to be the exact tool/weapon type, and you fully charged it, you have a split second to completely skip the attack charge whilst retaining the slot 2 weapons charge. Especially noticeable with attribute weapons whose attack speed attribute is set to be close to -100%, note that this is achievable fully without attribute modifiers.

Clearly an oversight, if i had to guess, game attempts to take the previous items/hands cooldown and apply its charging speed onto the new weapon/tool, until the game realizes that is not what is supposed to happen and continues with intended charge speed after some frames.

Note: for creative uses and attack attributes, this is very exploitable for slow attack speed high damage weapons, since either same weapon type can be used to get 100% charge easier and more consistently, or a near instant-attack speed item, e.g. empty or a stick, can be swapped from active slot back and forth to spam up to 50% charge meter worth of damage, albeit less consistently (would be used if the same weapon type is impossible to get). tl;dr exploitable in maps, minigames, etc. as an unfair damage advantage.

Also affects:
1.16.4, 1.17.1

Bug probably affects all versions since 1.9 combat update.