Filters (to filter out one’s own domain or IP range) were previously done in a batched job. Batching the filters meant it was possible to get notifications of activity that you did not want (e.g. the notification is fired off before the unwanted activity was filtered out).
Now the filters are run in real time as the visitor is taking action. Since these are now done in real time, if you create a filter, it will affect all visitors and prospects going forward. It will not, however, be retroactive.