Yes they probably would be slower. But we are not talking about V8s that are slower, we are talking about new "green" V6s that sound like leaf blowers being slower. The overall effect of this year's changes are happy engine manufacturers and a significant number of fans unhappy with what F1 has now become.
Your comment seems to give the impression that you think happy engine manufacturers isn't of great significance (please, feel free to correct me), but where would the sport be if all we were left with was Ferrari and Mercedes engines?
No other manufacturers showed any interest in getting into building F1 engines under the V8 regulations and one of the existing ones was unhappy with the situation. With the I4 idea floated, there were comments from a few new manufacturers (I think VW was one of them) about potential interest and Renault were fully prepared to invest in the new formula. Then we had the switch to turbo V6s and PURE started development, although ultimately that was unsuccessful and we are now awaiting the return of Honda. The previous situation could not last, as there just was not enough interest anymore. I'm not saying that V6s were the best solution, but we'd still be seeing problems if we stuck with the V8s.
You can talk up the new regulations all you like, unhappy fans equals less £££, and that is the only reason F1 exists. For that reason alone I am almost certain this year (and probably next) will be looked back on as the time F1 lost the plot. Hopefully someone will have enough sense to realise this before it's too late.
How would you suggest the situation is resolved?