Circuits to connect to Onion services (more accurate than onion sites, but is basically the same) are twice as long as those used to connect to “normal” websites (6 hops vs 3), so it’s normal for them to be slower.
Hours for loading is surely out of scale, though - also because the connection should time out much earlier. If an Onion services is so slow, the issue is probably in the server, not in the Tor network.