The Swans now face a huge battle to remain in the top eight after the loss on Sunday at the MCG, with games against St Kilda (ANZ Stadium), Geelong (Skilled Stadium) and the Brisbane Lions (SCG) to come.
Longmire said he briefly addressed his players after the game, but couldn't explain why their attitude had dropped away so dramatically from the team which fell only one point short of Essendon in round 20.
"Last week's effort was fantastic for four quarters. We didn't quite get there, and we all knew going into today's game what was on the line, and it was very clear," Longmire said.
"But if you looked at the two teams you would have thought Richmond was playing for a spot in the finals and not us.
"I said [to the players] it's just a very poor performance. There's nothing much else you can say.
"I guess you try and challenge yourself as a coach and then obviously the players about why we were in that frame of mind from the first bounce. Hopefully we have a think about it and come up with some answers on Monday."
The Swans, who have lost six of their past eight games, laid only six tackles inside-50 against the Tigers, and could not contain Richmond's influential half-back duo Brett Deledio (30 possessions) and Bachar Houli (32).
Longmire blamed a lack of skill and composure, and a willingness to work hard in attack, as reasons for the pair's dominance.
"Houli started the run and we tried to address him and then Deledio got going. We tried a number of things during the course of the game, but unfortunately none of them worked," he said.
"Our combination of poor ball use plus our lack of forward fifty pressure, meant they were able to slice us up on the rebound."
Longmire said speedy 20-year-old Gary Rohan is likely to return to face the Saints next week after two games in the reserves, but that effort - and not personnel - would make the real difference in the crucial clash.
"It was a big game today and we didn't respond. We have to respond against the Saints. The stakes are very high and we need to make sure we respond - that's our biggest challenge," he said.
"Longer-term, we need to get a lot better at our skill execution and our decision-making, but immediately we need to address our genuine hardness and want for the contest."
Follow Callum Twomey on Twitter at @Cal_Twomey
