Weird problem with empty CF

[Daning]

I have an app polling a few CFs (select first N * from CF), there were 
data in CFs but later were deleted so CFs were empty for a long time. I 
found Cassandra CPU usage was getting high to 80%, normally it uses less 
than 30%. I issued the select query manually and feel the response is 
slow. I have tried nodetool compact/repair for those CFs but that does 
not work. later, I issue 'truncate' for all the CFs and CPU usage gets 
down to 1%.

Can somebody explain to me why I need to truncate an empty CF? and what 
else I could do to bring the CPU usage down?

I am running 0.8.6.

Thanks,

Daning