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I've been wondering this too. And it's not the 11yo answer from the other comment: this is a more recent and noticeably slower delay that only happens in the northbound direction in recent weeks.

That doesn’t explain why it’s slower than usual.

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I've heard Harvard pays the MBTA to slow down under their campus because they have sensitive experiments and collections that the subway rumbling could interfere with. Maybe some new experiment just fired up?

Bicycle lanes

Given what we learned from the orange line shutdown, I have to imagine it's a slow zone due to repairs that haven't been done

lmao that's 100% an urban legend, another variation I heard from a Tufts student was because the foundation of widener library was so unstable that a subway train going too fast under mass ave would make the whole building shake. There's currently a much more severe slowdown than usual, but the normal slowdown is because the subway has to make a (relatively) sharp turn going between central and harvard. If you look at a map you'll see that the red line makes an almost 90 degree turn north to go towards porter square

It sounds like they're upgrading signaling on the Red Line. My speculation is that the northbound signals are temporarily out of service for maintenance, so all trains are required to proceed with caution, usually 15 mph on American railways.

Most sensitive experiments are done on air cushioned vibration isolating tables anyway since there are also other vibrations present from traffic, AC/heaters, people walking, and other normal every day processes

That's an interesting hypothesis. Almost 4 weeks of outage though?