With increasing number of slaves the performance of the build node column display seems to decrease significantly.
In our setup, after increasing number of Slave from 7 to 18, all views that are showing the "Last Build Node" column took more than double time to show.

For example a view containing approximately 70 jobs took about 15 seconds to show with 7 slaves and 55 seconds with 18 slaves.

Monitoring the CPU load (Prcoess Explorer and Jenkins Melody Monitor Plugin) shows a fully loaded CPU-core for the request.

After completely removing the "Last Build Node" column, the same view appears in less than a second.

The number of jobs obviously also has an impact (i.e. a view with 20 jobs and "Last Build Node" column shows in about 5 seconds). But the impact of the number of Slaves seems to be much higher.

I didn't test if the number of Offline versuse Online slaves has an impact.