I agree that the performance of the IETs when under diesel power, at least broadly comparable to an HST and often better. Under electric power it is significantly better.
My objections to them are not due to performance, but to the internal design, facilities and fit out.
I will let others argue over seconds or even whole minutes gained or lost under different conditions, by HSTs versus IETs. My concerns are more about what feels like a local, or at best, a regional DMU and not an inter-city service, rather than about the possibility of arriving marginally quicker.