Callum Hudson-Odoi scored twice, after mistakes by Tottenham's Archie Gray and goalkeeper Guglielmo Vicario, and Ibrahim Sangare completed the rout at the City Ground.
Tottenham had gone into the game buoyant after home wins against Brentford and Slavia Prague that had silenced some of Frank's critics, but he was again left trying to explain a return to the bad habits that have dogged his side.
"It was hugely disappointing. It's burning, annoying. That was a bad performance. Especially first half, overall we looked disjointed. We didn't win enough duels," Frank, whose side are mid-table with 22 points, said.
"It seems like we gave the ball away every time we won it back. We need to work to be more consistent. It's two steps forward and a step back today."
Forest's first goal arrived when Vicario played a short ball out to Gray who was dispossessed by Sangare who unselfishly squared for Hudson-Odoi to score.
Five minutes after the break, Vicario was caught out by a Hudson-Odoi cross that sailed over his head and in.
"That's down to individuals. We are in it together, we win together and lose together. It's hugely irritating that we perform like this today. Simple passing, I think we lose the ball 20 times, simple passes," Frank added.
"The better team won today. When things are going against us we need to keep doing the right thing and keep our cool heads."
Victory continued Forest's upward turn since Sean Dyche came in to replace Ange Postecoglou in October after the Australian had himself replaced Nuno Espirito Santo.
They are now five points clear of the relegation zone after winning four of their last six league games.
"Massive credit to the players the way they're absorbing what we're trying to do and activating it," Dyche said.
"There's still work to do but it's a much better league table. But it's got to continue, of course."