The Lilywhites were beaten 3-0 at Griffin Park and never looked like getting out of the game.
Neil said: "Naturally I'm disappointed, I didn't think we really threatened Brentford that much, we didn't have too many opportunities in the game.
"We found it really difficult to get in behind them and when we did concede possession, they certainly looked a lot more threatening than we did.
"When you combine the two it doesn't make for good reading for us - when we went forward it didn't amount to anything and when Brentford went forward, they were hitting the target and getting corners, applying pressure.
"It wasn't the type of performance I've had from any of my teams here, I was very disappointed in how we went about it."
North End fell behind in stoppage-time at the end of the first half, Ezri Konsa firing home from close range.
Referee James Linington had added on 13 minutes after Bees defender Julian Jeanvier suffered a head injury in the opening seconds of the game and needed lengthy attention before being stretchered off.
Neal Maupay made it 2-0 in the 54th minute, with substitute Marcus Forss scoring the home side's third goal seven minutes from time.
Neil said: "We started reasonably well but we didn't do the basics well enough, our use of the ball was quite safe and pedestrian, we didn't move it quick enough.
"We didn't damage them when we had the ball in good areas, our decision-making at times was poor.
"If you don't do the basics well, you give yourselves a mountain to climb.
"Brentford moved the ball relatively well but I have seen them play better as well."
"In the second half we changed to 4-4-2 and not much changed.
"When you come away to any difficult venue - Brentford are really strong at home - you need to play well.
"If you don't play well individually or collectively, then that is an issue, and today we didn't do either."