Wikipedia says that with unstabilised ylides they react very quickly in step 1-2 and step 4-5 is the rate limiting step. But with stabilised ylides step 1-2 is the rate limiting step and this is why stabilised ylides give E-alkenes. But there is no explanation as to why this makes E-alkenes and where in the mechanism it occurs.
I would have thought stabilised and unstabilised ylides would give E-alkenes to reduce steric hinderance but this is not the case. Any help explaining this would be very appreciated. :)