If you have lots of files in the source, then you shouldn’t use --no-traverse.
rclone can generally list a whole remote using a small number of API calls.
When you are using --no-traverse rclone will do a list API call for each file you upload.
When increasing the number of files you are copying, at some point --no-traverse will become less efficient.
In general if the source is more or less identical to the destination then definitely don’t use --no-traverse.