Why is ScreenCaptureKit throttled to about 7 fps?

I have an app that records a 32 x 32 rect under the cursor as the user moves it around and it sends it to Flutter.

It suffers from major lag.

Instead of getting 30 fps, I get about 7 fps. That is, there are significant lags between screen grabs.

This on an Intel Mac mini x64 with 15.7.3 and one display.

flutter: NATIVE: ExplodedView framesIn=2 timeSinceStart=1115.7ms gapSinceLastFrame=838.8ms
flutter: NATIVE: ExplodedView framesIn=4 timeSinceStart=1382.6ms gapSinceLastFrame=149.9ms
flutter: NATIVE: ExplodedView framesIn=5 timeSinceStart=1511.0ms gapSinceLastFrame=128.4ms
flutter: NATIVE: ExplodedView framesIn=7 timeSinceStart=1698.3ms gapSinceLastFrame=102.9ms

flutter: NATIVE: ExplodedView STOP polling totalTime=4482.6ms framesIn=28 framesSent=28 acks=28

Here's a testable excerpt:

import ScreenCaptureKit
import CoreMedia
import CoreVideo
import QuartzCore

final class Test: NSObject, SCStreamOutput, SCStreamDelegate {
  private let q = DispatchQueue(label: "cap.q")
  private var stream: SCStream?
  private var lastFrameAt: CFTimeInterval = 0
  private var frames = 0

  func start() {
    SCShareableContent.getExcludingDesktopWindows(false, onScreenWindowsOnly: true) { content, err in
      guard err == nil, let display = content?.displays.first else {
        print("shareableContent error: \(String(describing: err))"); return
      }

      let filter = SCContentFilter(display: display, excludingWindows: [])
      let config = SCStreamConfiguration()
      config.showsCursor = false
      config.queueDepth = 1
      config.minimumFrameInterval = CMTime(value: 1, timescale: 30)
      config.pixelFormat = kCVPixelFormatType_32BGRA
      config.width = 32
      config.height = 32
      config.sourceRect = CGRect(x: 100, y: 100, width: 32, height: 32)

      let s = SCStream(filter: filter, configuration: config, delegate: self)
      try! s.addStreamOutput(self, type: .screen, sampleHandlerQueue: self.q)
      self.stream = s

      s.startCapture { startErr in
        print("startCapture err=\(String(describing: startErr))")
      }

      // Optional: move sourceRect at 30Hz (cursor-follow simulation)
      Timer.scheduledTimer(withTimeInterval: 1.0/30.0, repeats: true) { _ in
        let c2 = SCStreamConfiguration()
        c2.showsCursor = false
        c2.queueDepth = 1
        c2.minimumFrameInterval = CMTime(value: 1, timescale: 30)
        c2.pixelFormat = kCVPixelFormatType_32BGRA
        c2.width = 32
        c2.height = 32
        let t = CACurrentMediaTime()
        c2.sourceRect = CGRect(x: 100 + (sin(t) * 50), y: 100, width: 32, height: 32)
        s.updateConfiguration(c2) { _ in }
      }
    }
  }

  func stream(_ stream: SCStream, didOutputSampleBuffer sb: CMSampleBuffer, of type: SCStreamOutputType) {
    guard type == .screen else { return }
    let now = CACurrentMediaTime()
    let gapMs = (lastFrameAt == 0) ? 0 : (now - lastFrameAt) * 1000
    lastFrameAt = now
    frames += 1
    if frames <= 10 || frames % 60 == 0 {
      print("frames=\(frames) gapMs=\(String(format: "%.1f", gapMs))")
    }
  }
}