Concurrent builds

[palewire]

It would be great if the observable build command could build dynamically generated pages concurrently. As far as I know, you can only build them one by one right now, which can take a good while if you have thousands of pages. Having multiple pages build in parallel could help reduce this problem.

[smndtrl]

Ran into the same problem where the "load" took 80ms per dynamic path + then execution of the data loaders for a few thousands is the same amount on top again.