Adobe Premiere CS6 has laggy preview with high disk access but low memory use
I am working in Adobe Premiere CC 2018, but if memory serves you should be able to still do this in your CS6 version. 

Set an out point 'O' at the end of the clips you want to preview. 
Go up to 'Sequence' in the upper left - and select 'Render In to Out' 

Once they're rendered on your timeline they should play with no lag.