Initially it sounded implausible that a small size increase could noticably raise connection establishment, but it's possible the small size increase causes MTU to be exceeded, and thus an additional roundtrip.
How many large tech companies do you think can do that? Each service wants to benefit from the use of other services. Ensuring there aren't dependency loops in this complex graph sounds tricky.
Firstly, congrats on having a popular project, caring about contributors, and seeking advice on how to handle this tricky situation.
When giving pushback, try to avoid raising the stakes, i.e. avoid public criticism, don't issue ultimatums.
If you have a private chat, that'd be great.
If not, try to state your concerns as plainly and non-judgementally as possible, from the perspective of your project.
"Hey X, I'm glad that you are and have been contributing so much to the project. There's a few different ways I see the program and codebase evolving, and I'm keen to work together on figuring out that right path.
It looks like there are places that have different visions, and if you could split PRs into smaller PRs, we can more easily find the changes that fit both our visions."
If there truly is disagreement on some PRs, you can be polite but firm, "I can see the value in X because Y, but this compromises Z, which is a core value of the project."
You have merge access, and should also consider whether this contributor is worth the hassle of all of what I just said. (In which case, skip straight to "polite but firm").
Twitter is a product that is seen as a digital town square.