This is a failed algorithm, I'm going to try with a simple approach.
Reworking how isPrime works, looks like a 3x speed improvement (work computer).
Changing True to False for composite numbers under 10. Oops.
Fixing off-by-one error that made isPrime report squares as primes. Again, oops.
Working problem 58. Completes in 5 seconds and change. Not too happy with this.