Recollect that the reactants are non polar, whereas TS1 has a dipole moment of 18.7D. Thus the barrier should be expected to be very sensitive to the solvent. In dichloromethane, the solvent I used in the above simulation, the value of ΔG starting from the complex formed from 2Br₂ and ethene (see discussion of those species at this review, DOI: 10.1002/chem.200390097) is 11.5 kcal/mol. Separating these three components to infinite distance raises this to 20.0 kcal/mol due to greater loss of entropy. These values still essentially correspond to a reasonably rapid room temperature reaction.

But change the solvent to tetrachloromethane (which is very non-polar), the free energy values rise to 22.5 and 30.8 kcal/mol respectively, which now corresponds to a slow reaction. The rate of reaction is thus predicted to be very sensitive to solvent.