Regarding lifting slow zones on the Green Line central subway tunnel post-monthlong shutdown:
I had a chance to ride the Green Line between North Station and Longwood Medical Area westbound, and eastbound back to Haymarket.
The slow zones between North Station and Gov't Ctr are towards Haymarket. I saw an S6 sign before curving into Haymarket, and then I saw 2 "RESUME SPEED" signs in quick sucession past Haymarket. The second "RESUME SPEED" sign appeared to be shortly before the merge from the Gov't Ctr loop. So it seems the 5 MPH WB speed restriction between Haymarket and Government Center doesn't affect Boylston St. streetcars looping to return to Park St.
The trip from North Station to Longwood Medica Area was extremely slow. The streetcar left North Station at 13:47 (1:47 p.m.), and did not get into Longwood Medical Area until 14:18 (2:18 p.m.), a 31 minute trip that
used to take 20 - 22 minutes back in May 2018.
Much of the central subway tunnel still seems to be capped at 15 MPH (25kmh) from Government Center to Symphony. There may have been some S25 (40kmh) speed limit signs somewhere near Gov't Ctr, or Arlington, but I couldn't see them or tell them apart from the S15 signs in the dimly lit dark tunnel. On curves and switches in the tunnel between Government Center and Symphony, they are limited to 6 MPH (9.6 kmh) with the speed limit signs stating as such.
At Northeastern there is a 10 MPH (16 kmh) limit at the switches, then after that it is S25 with no additional speed limit signs before Longwood Medical Area.
Heading eastbound there was too much rush hour crowding so I could not observe any speed limit signs, the trip was much faster and I got to Haymarket from Longwood Medical Area in about 23 minutes (historically around 18 minutes in May 2018).