The primary reason steroid hormones usually act slowly is that

Because steroid hormones bind to the intracellular receptors and can regulate gene expression (turn the genes on or off).

Explanation:

The receptors located in the nucleus are called intracellular receptor (unlike transmembrane receptor which is located on the membrane). Usually, the ligands of intracellular receptors are small, hydrophobic molecules, since they must pass through the plasma membrane to get to their receptors. For example, hydrophobic steroid hormones, such as estrogen or testosterone are ligands for intracellular receptors.  

Intracellular receptors have DNA-binding activity, meaning they can attach to specific sequences of DNA and regulate its gene expression (e.g. transcription).

So, it takes time for them to perform their function.

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