SubAlert polls followers every X seconds for new entries, as there is no live-notification sent out via chat on a follow (as there is on a sub). So if someone follows right after the polling cycle just tripped, it can take a bit (eg: until the next time SA polls the follower list).
TNotifier follows a similar polling method, and while it may be a shorter poll interval, a 1-2 second delay is just being lucky and having hit 'follow' right before it checked the list. Far as I know, Twitch hasn't come up with a push-API as of yet, and over-polling can be a concern.