Some of these aren't quite apples to apples comparisons though. E.g. Debues-Stafford. She went 4:03 to 3:56, but that leap was also her first year with a pro training group after graduating college. Someone having a big jump when they turn full time is pretty standard. If that someone is someone who ran 4:03 at 21/had already made an Olympics and a world semi final as a junior *and* now they swap out college teammates for Laura Muir? Yeah not surprised to see a big jump. I actually bumped into Lynn Williams - the prior Canadian 1500m record holder shortly after Doha 2019 and asked her about her record being smashed that year. She said she wasn't surprised at all and had been saying since FISU 2015 that Stafford was going to break 4. That's different to Sifan running 3:56/57 many years straight before joining a coach with a shady reputation, and exploding to 3:51.
But also have to think about times almost logarithmically. The faster you get, the harder it is to improve. I.e. to jump 4:20 to 4:10 is exponentially easier than jumping 4:10 to 4:00. The jump 3:56 to 3:51 is a harder jump than 4:01 to 3:56. Then when you consider the 3:56 PB for Hassan pre 2019 was getting dragged by Dibaba, while her 3:51 had no pacer, was front run start to finish, and was following semis, heats, and a 10000m where she closed in 3:59. To me, even without career context that's actually a larger jump in fitness than going say 4:03 to 3:56.
I want to believe in her because I find her actually quite likeable: She's got a good story, and is a fantastic interviewee. If I could sit down with any athlete in current track and have a good three hour conversation with, she would probably be top 3. Unique set of talent, experiences and personality. I agree, I do personally find Tseguy more suspect though Jo. Hassan ebbs and flows - as she focuses on strength like right now her speed fades a bit (2:01 800 last month) while Tseguy is apparently still knocking everything out the park 800-10000 all at the same time.