I meant to post about this, but didn't get around to it.
This study shows why the Delta variant is so much more transmissible than previous variants.
Specifically, the viral load of infected is 1,260x higher at first detection compared to the wild type virus (each Ct level is a doubling of the viral amount). Time from exposure to first positive test peaked at only 3.71 days vs 5.61 days for the wild type. Suggesting that Delta has a shorter incubation period than the wild type and people are more infectious. This is likely why Delta is believed to be ~40% more transmissible than Alpha.
This could be a double edged sword overall. If people are more infectious and the incubation period is shorter, there will be less time for pre symptomatic transmission than before. At the same time people will be very infectious during this window. Testing after exposure and isolating upon becoming symptomatic are as important as ever.
Delta seems to have different symptom patterns than the wild type virus. People are reporting mostly headaches, sore throats, fevers, and runny noses in the UK data. Runny noses were pretty rare symptoms with previous strains.