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How does one know if upgrading OS will slow down their relatively older machine?

For example I bought a MacBook Air a couple of years ago. It came with Leopard and I've already upgraded to Snow Leopard, but now want to download Lion. I'm concerned though that it may slow down my machine.

I've heard of this before where upgrading OS slows down performance on older hardware.

How can one know if they risk a slower performing machine if they update OS?

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up vote 3 down vote accepted

Lion seems to use a lot more memory than Snow Leopard. I had to upgrade my 2 year old iMac from 4GB to 8GB of memory, and it still feels slow sometimes. You may want to avoid the Lion upgrade if you have less than 4GB of memory, and definitely if you have less than 2GB.

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I noticed a slow down on an antique 2007 MacBook with 2 GB RAM. Lion has a lot of bells and whistles which will drag on older machines (flies on modern Macs though). – dan8394 Dec 3 '11 at 18:05

I upgraded like you from leopard to snow, (on a core duo(not core 2 duo)) MacBook, It ran a lot faster.

I have also upgraded an Intel core 2 duo from Leopard to Lion and it has also gained in startup speed, effictively it streamlines basic processes.

I am led to believe (correct if wrong) that it uses the same processing power, but in a different way (hence only compatible with core 2 duo and above.

As MacBook Airs were introduced to i processors this year you have a Core 2 Duo - which will run at a faster speed that Snow Leopard.

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