Exactly, If I were using node and needed to calculate primes I'd probably write some C++ code to handle that part and run it as a server in a separate process that can be called asynchronously from node.
Hopefully you'd try another algorithm before you did that ;). Primality tests are one of the most highly understood and well documented areas of CS - trial division has been obsolete since, what, 1640?
Node wasn't ever really meant to be used for CPU intensive tasks - especially the ones like this which you can fairly easy parallelize.