“Slow zones” on the red line have escalated significantly recently. Don’t know all the details, but guessing it’s related to issues with the tracks themselves.

https://twitter.com/friendchristoph/status/1628026370054922245?s=46&t=XlSuTmJiStPgJ5Aq846MFg

The MBTA has not been very transparent about this. 3rd party trackers and advocates have been making most of the noise about this issue worsening in recent weeks and months.