That kind of depends on what you do inside the trigger and how often it is fired. Trigger run synchronously. So, everything will wait to wait until the trigger finishes. If you have complex logic inside the trigger or you're even calling external processes (many people think it's a good idea to send mail from triggers, for example), you're likely to hurt performance. Think carefully if you really need a trigger or can achieve your goal otherwise and if you need a trigger keep its logic as simple as possible.
If you need to call external processes "from a trigger", it's usually a good idea to let the trigger log the necessary details in a working table, then set up a job that checks the working table for new entries, and launch the external process.