Workplace has limits in place to prevent abuse of our features and to protect people from spam and harassment. For example, if someone sends out several messages to people they are not connected to, they may be warned or temporarily blocked from sending messages.
Additionally, for example, you shouldn't tag too many people too frequently on notes, as that may be considered spam.
Limits such as these are based on different factors, like speed and quantity, but we can't provide additional details on the rate limits that are enforced.