With Global Entry, that shouldn't be a problem, at least if you know how to use your elbows.
Most international flights come in late afternoon/early evening and around that time it can get pretty full with immigration lines backing up into the aisles sometimes even back to the gates. Typically the handler sorts people by US and non-US with one group at the left wall and the other group at the right wall. You need to mercilessly plow through there yelling/mumbling/singing "Sorry, Global Entry", "Global Entry coming through" "Global Entry, please" and ignoring the dirty looks and hissed insults.
In extreme cases, they will hold people on the plane and you are not allowed to disembark (Global Entry or not) until there is enough space in the aisles to take more people. However this is rare and should never be more than 15-30 minutes.
For the following I'm assuming that your BOS -> DC flight is on a different carrier with a different ticket.
International arrivals come into Terminal E and once you are through immigration and customs you probably will need to switch terminals. For C I would just walk unless you have sizable luggage. For A and B, take the shuttle bus. If you can check in online upfront, I would do this, otherwise check in at a kiosk and drop off your bag and then off to security.
TSA PreCheck tends to work pretty efficiently so that shouldn't be much of a problem.