There is a notion out there. I am not saying you. But some people think that the goal of training at low heart rate like we do is to develop it so much so that we can also race with low heart rate. The only race that we recomend be done around our low hear rate threashold we call MAHR is an Ironman. And in that you can even go a little higher if sustainable. But for all other "races" one will more that likely race much higher than MAHR. Run races in particular even marathons can be raced above MAHR.
So as you can see it will be very tough to race and always train below MAHR.
Doing speed work is an important component to be able to race well. It even helps your aerobic fitness. There are planty of research done on the benefits of doing speed work not only on anaerobic fitness but also aerobic. But of course there are negatives which is why we only do speed work for some weeks before a big race. So you develop you base, aerobic fitness and improve your pace below MAHR. but at some point you need to introduce speed work before your big race and later taper to then have your best result. That is the general idea. I hope it helps you.