nodetool ring runs very slow

[Feng Qu]

We noticed that nodetool ring sometimes returns in 17-20 sec while it normally 
runs in less than a sec. There were some compaction running when it happened. 
Did compaction cause nodetool slowness? Anything else I should check?
>>> time nodetool -h hostname ring
....
real 0m17.595s
user 0m0.339s
sys 0m0.054s
 
Feng Qu