The description of the Slow spell states that:
On its turn, [a creature] can use either an action or a bonus action, not both.
Haste gives an affected creature a second action. Assuming we're not talking about taking the attack action again (since slow specifically limits you to one attack) does slow limit your actions to only one?
Best Answer
TL;DR
Many of the effects of haste and slow negate each other, with the following exceptions:
There are some specific effects, but they're unambiguous so I'm leaving them out of this description.
Let's examine both in detail:
...
So we can see that many of the effects are cancelled for both spells, specifically:
This leaves two components that we can break down:
With haste, the additional actions can only be:
With slow, you're restricted from doing certain actions:
From this we can gather that being both hasted and slowed will basically cancel each other out with a few restrictions:
Addendum
Note that you have advantage on Dex Saving throws due to haste, but -2 on those rolls due to slow.