FileMaker Network Sharing is very sensitive to network latency, which is the time it takes a request to get from one end to the other of your connection. Things like summary fields, unstored calculations, lots of portals, etc. tend to run slower when going out over a WAN like the internet vs. running on a local LAN.
Note the quality of your network connection will hence greatly impact your performance. You may want to test out a FileMaker hosting provider, like ourselves, who generally have better network connections (our connections are second to none), which may improve your performance:
Our accounts include a 30 day trial, whereby if you cancel for any reason within the first 30 days you owe nothing. So, signing up for an account and trying it out for a month is a risk-free proposition.
Otherwise, I would recommend minimizing your usage of the aforementioned features as much as possible if you'll be running over the internet.