Locke's hopes that his charges would build on their League Cup derby win over Hibernian in midweek were shattered as Hearts appeared to lack spirit after going behind in the first quarter of the game.
"It's really disappointing because the way we played during the week, we deserved to win the game," Locke said.
"And if we put in a similar performance we would have probably taken something out of that. That's the disappointing thing because we have to maintain that level of consistency and we don't seem to be getting it at the moment.
"One of the things is, when we went a goal down, we can't let our heads drop as badly as that because there were still 70 minutes left to play.
"The heads were down a wee bit at half-time but there were still 45 minutes to get back in the game and the next goal was crucial. We never started the second half well enough and it's cost us."
St Johnstone counterpart Tommy Wright feels the front pairing of Stevie May and Nigel Hasselbaink is paying dividends, with both finding the target against Hearts.
The midweek win over Morton is the only game in four that Hasselbaink hasn't scored, while May has taken his tally to nine for the season.
Wright said: "He is disappointed he didn't score on Wednesday because I don't think he has scored three in a row. It's a nice wee partnership and Nigel is a handful.
"When you get the ball into his feet in that last third, defenders are wary of him. He has really good feet and good body strength as well.
"So I'm really pleased with Nigel. It's a partnership that has blossomed over the last couple of games."