Hi guys,
I do around 35,000 miles per year and 90% of that is either towing around 3-4000 lbs for work or 2 f18's & associated stuff.
IMHO the 8-10% theory is ok up to 150lbs. Most car tow hitches are designed to have max 200lbs vertical load and up to 5000lbs gross trailer weight.
As for speed - if your trailer is loaded properly and the load tied down, speed is generally not an issue. (Have been guilty of making a TheMightyHobie18 do 120mph down a fwy late for a regatta many years ago
. Common sense needs to apply though - you can't go around corners anywhere near as fast and stopping distances are longer.
Tiger Mike