I get that that's what the headline says, and what Inrix says, but if you read the article you'll find out that "worst city for traffic" is not at all what the data means. Inrix compared peak hour traffic to off-peak traffic as their metric for congestion. As Adam Vaccaro notes in the Globe, that means that LA, with its all-day congestion, ranks 6th, not 1st, as it used to - the difference in rush hour isn't as large as it is here.
What Inrix has actually done here is measure peaking factors. That doesn't mean traffic in Boston isn't bad (it ranked 7th last year and presumably would have been 6th, 7th, or 8th this year if they hadn't changed methods), but I'm genuinely confounded at why Inrix would have (A) seemingly screwed up their methodology from something relevant to something irrelevant, and (B) made false claims about what their new dataset represents.
That doesn't even get into the issues that Vaccaro alludes to with using 65% of free flow speed as "congested". On a Boston city street, rush hour brings a lot of things that aren't cars - more bikes, more people crossing with or without crosswalks, more buses pulling over to stop, school buses, etc. In sprawling cities where all the major roads are 4-6 lanes wide, these things don't slow you down as much as they do here. Boston is essentially penalized for having local roads that aren't car-centric.
Beyond all of that, as Vaccaro also notes, the use of speed as a metric doesn't necessarily scale up to longer commutes, since Boston is more compact than other cities and people may not be covering as much distance. They might be going slightly slower, but they aren't taking all that much longer to get there.