Do you have actual numbers handy? The point is that you only have about 10 mins to work on a block. The longer it takes to compute collisions the less benefit

Do you have actual numbers handy? The point is that you only have about 10 mins to work on a block. The longer it takes to compute collisions the less benefit you get from them. That is unless you precompute the collisions using irrelevant transaction data. The collisions must be recomputed everytime one of the transactions used is accepted on the blockchain.