If you're at the size where slow CI negatively affects your projects, then you're big enough to own your own CI (at least the build agents).

Strongly disagree. When I joined my current company, as an early engineer on a fairly new product, we had an 8 minute deploy time and I see that as a fairly critical component to our rapid iteration cycle that was critical to the company at that stage.

We had 3 engineers and had no time to spend on owning our own CI.