I would argue the problem is for all the CI systems out there at the moment, there all "stupid": i.e. none of them try to predictively spin up an environment for a dev who is committing on a branch, none of them have any notion of promoting different "grades" of CI worker (i.e. an incremental versus a pristine) and none have any support for doing something nice like "lightweight test on push".
All of this should be possible, but the innovation is just not there.
> notion of promoting different "grades" of CI worker (i.e. an incremental versus a pristine) and none have any support for doing something nice like "lightweight test on push".
Which ones don't support that? Anything with a Docker cache, for example, can build layers efficiently, reusing previously built layers. Build triggers let you choose when to start a job, so GH actions, for example, can trigger a full test on any PR and a light test on any commit to a branch.
All of this should be possible, but the innovation is just not there.