well, i live here so for what its worth…
it hugely depends what time of day, and cornwall at the moment seems fuller of cars than ever before.
a30 as you come off the m5 at exeter is a sticking point if we are talking peak times/friday/saturday. after that its free and easy as theyve eliminated the single lane bit up on bodmin. however it all then comes to a grinding stop at the garland cross roundabout and tails back for many miles. once you clear that its then single lane to chiverton cross roundabout and every single day now its a crawal that gets slower and slower until you que for a long way to said chiverton cross.
an option would be to head into truro from garland cross then go down on the falmouth road to the south coast.
if you decide to stick with the a30 i would consider coming off at camborne and going cross country to helston.
but if you dont then once you get on dual carriageway again you will see signs for camborne west and then you will queue again to the hayle roundabout. once past that you will then queue again to the st erth roundabout. once clear of that you will then queue miost of the way to the marazion roundabout. when there turn left and hope you don’t get stuck behind a tractor, large motorhome or tourist incapable of driving on a bendy road faster than 20mph all the way to helston.
more or less, cornwall is utterly foul to drive in just now and every trip takes way longer than it does when all the buggers go home.