Puppies often inhale their food - just because they can. I wouldn't worry too much about how fast it goes down! Just leave him to it. Mine could clear a bowl in nano seconds when they were little.
I also wouldn't rely on toast or anything at this stage. These dogs physically grow so fast that they need decent nutrition to build strong bones. And you will probably regret teaching him at such a young age about human food and the consequential begging every single breakfast time. Because that's what he'll do!
The only advice I'd give is be guided by your dog. If he's looking skinny, slightly increase his food. If he is acting as though he is starving to death, being obsessed by food and so on, give him a bit more to eat.
If he is looking like a barrel on legs, decrease his ration a bit until he evens out.
These quite big breeds do go through some very odd growing stages - leggy, plump, hot rod but you can feed by eye to a certain extent with all of it.
BUT - and you have probably already worked this out, labs and retrievers grow at a rate of at least an additional kg a week, remember to increase the daily ration to accommodate that. It's easy to forget how much bigger they grow in how little time.
Try increasing his ration a little if you think he needs it - rated that than toast, tbh. A raw carrot is a good thing for a teething pup to nibble on too.