Are you importing directly into the map or adding to My Content (with a organizational account)?
If you're importing directly into the map: When using latitude and longitude, the first 1,000 rows of features are displayed on the map. When using addresses, the first 250 rows of features are displayed. (the 250 limit is for public accounts {free}, but you may get up to 1000 if you have an org account, I'm not sure). While you can break it up into smaller files, adding over 1000 features across several layers will slow down browser performance.
Thanks,
Mike