I do not know how Java handles this, but maybe it actually enables exceptions for underflow which invoke some handler.

Otherwise I cannot see how you can obtain such a huge slowdown, unless your code consists entirely of back-to-back operations with denormals and of nothing else.

I am not sure what you mean by "state variables", but if they are pushed into the denormal range, they should be changed to double, not float.

If you push double variables into the denormals range, then it is likely that the algorithm must be modified, because this should not happen.

Underflows, i.e. denormals, are difficult to avoid when using float variables, which can be mandatory in DSP algorithms for audio or video, but outside the arrays processed with SIMD instructions at maximum speed, the scalar variables can be double, which should never underflow in most correct algorithms.

For computations run on CPUs, not GPUs, only very seldom there can be reasons to use a scalar float variable. Normally float should be used only for arrays.