Here's the scoop -- Billie called out musicians who do 3-hour shows in a big way. She said it's "literally psychotic" during a Stationhead Radio interview on Thursday ... and she added, "Nobody wants that. You guys don't want that. I don't want that. I don't even want that as a fan."

Billie doubled down, saying, "Even my favorite artists ... I'm not trying to hear them for 3 hours. That's far too long!"

Billie didn’t name names, but everyone's fans are buzzing -- The BeyHive thinks she's referencing Beyoncé's "Renaissance" tour, but the general consensus is it's a diss of Tay Tay's marathon-length "Eras" tour shows.

However, in a recent interview with LA Times Billie showed praise for Taylor saying, "I find it really hard to play stadiums. Beyoncé and Taylor [Swift] are untouchable superstars; the fact that they can put on a show that long, and it’s filled with so many incredible moments, is really amazing."

The thread in question was captioned, "Taylor Swift breaking her neck doing everything to prevent other artists from shining" ... and referenced Taylor dropping 3 new versions of her "The Tortured Poets Department" album on the same day Billie released her new album last week.

As we reported ... their dueling fan bases were dissecting what Taylor's move meant: Some said it was just a coincidence, while others suspected it was strategic.

You'll recall ... Billie first kinda came for Taylor a month earlier, publicly calling out artists who constantly re-release their albums on vinyl -- a move Taylor's known for pulling.

Billie later said her critique wasn’t aimed at anyone in particular ... but it looks like it might have been the opening shot in what could be one of the most intense pop feuds ever.