Just remove burst speed all together.

It is a highly frustrating mechanic. You can do whatever you want with no counter play if it is burst which is BS and inherently makes any card that isn't burst speed tremendously worse.

I get. Its a way to have a different type of counter play outside of having a bunch of counter spell type cards. But the difference is that playing another card to counter a spell feels good. Like you out played them. But playing a bust spell just feels cheesy overall. One the other person can't counter it and for the caster it requires no forethought as to what the other player may do to counter it and have to ask themselves how do I counter their counter.

Burst is a completely brainless mechanic that is frustrating for the enemy player and encourages brainless play from the caster.

Burst speed is definitely one of the mechanics in LoR that makes it much harder to balance than a typical CCG, but it's also one of the ways in which LoR sets itself apart from other CCGs. Certainly you need to know what burst spells your opponent is likely to be playing, and plan around them differently than you might for slow or fast spells, but that doesn't make it inherently unfair.

I also think it's not basically the opposite of a "brainless mechanic" as you describe it - if anything, it makes timing your plays so much harder than other games (like MTG) that even seasoned CCG players regularly come out of interactions in LoR feeling a bit dumb about how they sequenced their plays.

Burst are usually buffs. You can't react to the buff itself, but you can play cards acordingly afterwards.

For example, if your 2/3 is about to trade with my 3/2 and I play an elixir of iron, you can't prevent it from turning into a 3/4, but you can play something like elixir of might, so my unit dies too or glimpse beyond or something to deal 2 damage. Counterplay still exists.

I think burst is fine as long as it's restricted to buff/debuff cards. A direct damage burst would be completely broken.