> I did notice that my debian-based system got noticeably slower and unresponsive at times the last two weeks, without obvious reasons. Could it be related?

Possible but unlikely.

> I read through the report, but what wasn't directly clear to me was: what does the exploit actually do?

It injects code that runs early during sshd connection establishment. Likely allowing remote code execution if you know the right magic to send to the server.

Are you on stable/testing/unstable?

With our current knowledge, stable shouldn’t be affected by this.