You can verify this yourself by simply looking at the expression for a period of oscillation for a simple harmonic oscillator. The period is given by
[a factor of 2pi hanging around somewhere]
where T is the period, k is the spring constant (a measure of stiffness of the spring), and m is the oscillating mass. You can already see that to maximize T, you want to make k as small as possible, and m as large as possible.
Zz.