lactulose versus miralax

I find lactulose works faster (within 3 hours when taking 50 ml) than miralax, which takes a few days. Lactulose also seems harsher, but works better.

I thought lactulose and miralax were the same kind of drug. So why does lactulose work faster?

If it's dosage, can I take a high enough dose of miralax - which tastes much less bad - to replicate the effect of taking 50 ml of lactulose?

lactulose versus miralax