Xcode 15 ignores EnableBuildSystemIntegration flag

From observations made on our project, it seems Xcode 15 ignores EnableBuildSystemIntegration flag and bahave as if it was set to true. Compilation summaries in comparison to Xcode 14.3.1 are quite explicit in this (please observe number of tasks, to notice the behaviour changes).

Xcode 14.3.1 (EnableBuildSystemIntegration ON)
SwiftCompile (2473 tasks) - 1844.916 second
Xcode 14.3.1 (EnableBuildSystemIntegration OFF)
CompileSwiftSources (296 tasks) - 1323.457 second

Xcode 15.0 (EnableBuildSystemIntegration ON)
SwiftCompile (2473 tasks) - 2269.863 second
Xcode 15.0 (EnableBuildSystemIntegration OFF)
SwiftCompile (2473 tasks) - 2200.645 second

Xcode 15.1 reproduces the same.

Setting EnableBuildSystemIntegration to 0 (OFF) was a big local performance boost, that we lost (e.g. incremental build on core modules is ~30% slower). On top of build time degradation related to EnableBuildSystemIntegration, we observe Xcode 15 being slower all around for a highly modularised project like we own. Can someone confirm the issue with: EnableBuildSystemIntegration on Xcode 15? Does anybode else observe slowdown with Xcode 15 and got any remedies?

Also observing this, Xcode 15 is very noticeably slower for both clean and incremental builds.