There are challenges types that are no time limit, it's one marked as a crown on the map.
Lines and vert challenges will always have a time limit
Competitive modes such as Block Party and Spot Battles have a time limit because it's against other players.
When it comes to the servers getting better it depends on your connection to EA servers most times or if there's upcoming maintenance to start a new season. The time limits are there for a reason if the connection isn't stable it's the handshake in between your connection and EA. If anything that probably needs to be looked into.