When the tracer is running it creates a file to store info of primary page and param page at every step. The tracer becomes slow as the file size grows. Sometimes it might even bring down the server if all the server space is used for tracer file. To over come this try to 'Abbreviate events' if you don't was page data or trace small portion of code.
Remember the tracer file does not get deleted unless you close the tracer.
Go to your windows 7 (assumption here) task manager and click on the column header of your "Memory (Private Working Set)" column to sort by size. If your browser window that is displaying the tracer has become too large, that may be the issue.
When that occurs for me, I try tracing less. For instance, I sometimes try tracing activity enters and exits but not the activity steps themselves.
Or, I trace only a particular activity (and allow the steps to be shown). For pre-Pega-7 versions, this was called "trace open rule". In Pega 7, it's done in the actions menu for the activity rule form.