Using 1080 in 720 Sequence hurts RT

i shoot in 1920x1080 , edit in 1280x720.  because number of 'interview' projects need deliverd in 720, allows me 40-50% upscal on interviews (basically closeup , medium shot without having change camera during shooting)  in fcp never had rt (real time playback) issues doing this.  have found in cs5.5 when this, every single clip needs render because it's being scaled.  if second layer b-roll gets worse.  (note, dropping in single clip gives me red bar , if place b-roll clip on track 2 on interview, stutters , pauses).

 

here's question.  working on following system:

 

macpro 4,1 (2009)

os  10.6.7

2x 2.26 ghz quad intel

20gb ram

geforce gt120

 

system & software on internal 7200 drive #1

media on internal 7200 drive #2 (not raid)

project on inernal 7200 drive #3

 

is there can short of complete new system improve rt performance of using 1080 footage in 720 sequence? 

 

while know of following system, of following alleviate problem.  if 1 , how (or recommended product) each?

-seutp raid0 ?

-faster graphics card?

-more ram?

 

is there mac system make work without having render every clip? 

 

will premiere pro next address issue?

 

thanks

ally

a graphics card allows gpu acceleration trick.

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