How many objects are in the room? More that the raw size of the room. what causes slowdown is the amount of objects taking up memory.
That being said maybe there's a size where rooms start to chug, but I havent experienced, and there's games out there with pretty big levels.
If you aren't already, you should call most of your dynamic objects through code, instead of placing them in the room through the IDE.
A simple way of doing this at the start of a room would be
GML:
//On your game controller's room start event
if (asset_get_index(<roomName> +"_init") != -1)
{
script_execute(<roomName> + "init");
}
Inside the _init script you create all your instances and set up the room as needed.