When running rosetta at background, I found my 3D MAX rendering speed slow down a bit. From 31s to 33s at a simple scene.
It's about (33-31)/31 = 6% slower down.
This test was run on my PC which has Prescott 3.2G CPU, 2G DDR2 Memory. Two rosetta threads echo consume about 100MB memory.
I think it could affect system more seriously if you only have 512MB memory or less.

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In general preferences, try setting 'Do work while computer is in use?' to No.

Inevitably, if you have several programs running that use large amounts of memory, even though some are at idle priority, when they get a timeslice, the system will spend time in kernel mode page swapping etc., this will impact the performance of the system as a whole.

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Wave upon wave of demented avengers march cheerfully out of obscurity into the dream.

yeah - there will be some system impact, but it's minimal, although it'll be more significant on computers with less RAM and smaller caches etc. Of course, it'll have no effect for the majority of computer use and I'd expect it to only be noticeable when the computer is being maxed out variably, as Rosetta will keep re-filling the caches between peak loads of the game's thread.

If a program that requires longer bursts of peak CPU use to do some heavy processing (i.e. >1s), like most office apps such as Excel, I doubt rosetta would make any difference as Excel would fill the L1 & L2 caches with its data for the duration of the heavy CPU load and rosetta would be effectively paused.

Having said that, on some of my low-end remotes (Pentium 3s) I've set rosetta to not do work while the computer is in use, although I'm sure most of the time it'd not be noticed. Just make sure you've got 'leave in memory' set to yes!

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