I love this question, similar to brainstorming I've been doing this month. I run 5-6 days a week, 5-15K. I came back to biking this month, doing 2 group rides and one solo per week. None of the above would be considered particularly fast, but they tend to be 2-3 hour rides. The added biking has slowed down my run tempo, there is just no doubt about it. My stride doesn't groove in until mile 2 or later and it seems I've lost top end.

So, I skipped the biking for a week and the runs have been improving. So it leads me to think that the more compact nature of the biking spin compromises that running stride, plus the run warm up seems to take a good deal longer the day after a bike ride.

Hardly scientific, but if I was really serious about my run times I think I'd limit my bike time.