yeah thats a major thing thats certainly not mentioned much is the delay most forums have between sending messages, frankly you could send more PMs by copy pasting usernames and copy paste your message because manual sending on forums you can send anymore where 3-5 per message

pmblaster=1 per minute or 2 per minute
manual=you could techincally send 10 or more if you setup usernames in a txt file in increments of the boards limit

if pm blaster took in account this it might be more effective to have it put in 3 or 5 usernames per msg depending on the board