| Yes begonias are slow to get started, even from tubers. They just seem to sit there for a while then all of a sudden they seem to spurt up. Yes I would wait until they get their second true leaves. I would off set or raise slightly your humidity cover so the plants get some air circulation and it will still keep in the moisture. However when they are getting close to all having their second leaves I'd get rid of the cover. Since you don't have lights or greenhouse, I'd move them outside in the day when the weather is warmer and bring them in at night. Put them in a protected spot in shade. If you leave them on the window sill they tend to get leggy and lean towards the sun. | 
