For the loading time, in the first pinned post:

Very long loading time on worlds?
The first time you launch an old world in Hearth & Home, the client will do a “generate locations” command automatically and it will generate all those new locations we have put into the game for you players to discover. This can however have the issue of moving already spawned boss locations so the boss-marker on the map can be wrong. This is solved by removing the marker and interacting with a Vegvisir stone again.

As for the lag, my guess is a server issue? Normally older worlds should be 100% compatible from what I've heard (except if you explored the entire map, in which case new assets won't be generated)