I can't understand derogative reviews of this movie. It is not worse than 85% of horror flicks made in other countries. It has a mystery, mood, suspense, some blood, a lot of family drama and quite a satisfying, explanatory endings. Definitely not slow and I'd rank it between 5 and 7.
A couple who have lost their 6 year old son (he was never found after a walk, but in the end of the movies we do understand what has happened to him, won't spoil you). They decide to adopt a child from childcare monastery, an abused, very weird boy who immediately starts to transform into their lost Vanya. But of course things don't go easy. He is wild, violent, the wife starts sees visions of the presumably dead Vanya and then she gets pregnant and the adopted child is a threat. Meanwhile her husband tries to understand ho the adopted "Vanya* really is and who was his mother, as he seems to never get old. and possesses quality of transformation of anything we lost or grieve about..
It is not a bad example of Russian horror movies and yes, Omen is more boring. Give it a try.