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I have a Dell Precision 3520 with the following specs regarding to RAM:

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Slot 1 is filled (see below), slot 2 is empty.

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In some sites RAM is presented to upgrade which is barely available, so I like to buy RAM from some local dealer. My questions are:

  1. If I buy DDR4 16 GB RAM with a clock speed of e.g. 2400 or 2667 MHz, will it decrease the speed of the already placed RAM (1200 MHz)?
  2. Would I benefit if I buy two 2400/2667 MHz RAMs? (as maybe 1200 MHz is the maximum that is used by this 'old' laptop? (is there a way to know the best speed to use for this laptop? or is faster 'always' better?)
  3. If the latency of the new RAM would be e.g. 19, would then the 'overall' latency be 19 ? or worse than 17 or 19 ?
  4. If I buy one new 16 GB RAM and find out my system is much faster, if I would buy another same type of the new 16 GB RAM, is that equally good as buying a 'set of 2 RAM modules' ? Or can I assume two equally RAM modules would behave like a set?

The RAM I have in mind is a Crucial CT16G4SFD824A (16 GB DDR 4 SODIMM 2400 MHz, CAS latency 17, voltage: 1.2V, ECC Type: Non-ECC or: Corsair Vengeance CMSO16GX4M1A2133C15 (16 GB, DDR4, 2133 MHz, CAS LAtency 15, 1.2V, Non-ECC)

There are more types, but just some to show as example.

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So the first effect is the RAM would run at double the speed. This will give you a performance boost of 0 to 50% (depending on the workload). Also your RAM capacity doubles so if you are now running low on RAM, you could be tripling your performance (if I'm being optimistic).

Now let's look at your concerns

  1. The BIOS will choose RAM settings that are based off of the JEDEC settings and will in theory choose the fastest one that is on all modules. If you buy the Crucial stick, this will probably mean both sticks will run at 2400Mhz.

  2. See 1. Since it will take the fastest one compatible with both modules, it's probably better to get the same speed for there to be a bigger chance of it picking the wanted speed.

  3. The BIOS may adjust the speed to fit the slowest stick. This means if you mix CL17 and CL19 RAM it would probably run at CL19.

  4. Please don't do that. Your laptop probably only has 2 SODIMM slots and getting 3 sticks means you have to remove one and that won't give much of a performance boost. If you don't have one, spend that $60 on an SSD, which will give you a mayor improvement in OS/app loading time.

From the example sticks you have shown me, the Crucial RAM fits your current RAM the best, so that will be your best bet. It's also $6 cheaper where I live.

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  • Thanks for your answer, however, to clear the fourth question: my issue is that I already have 1 SODIMM, and to prevent buying two, having to keep one in the drawer for nothing, can I try buying 1 new, and if it works, not buying the second new one (of the same type), or should I buy them as 'pair' as two new ones but not in the same batch will work less optimized? (also wondering how I know there will be much benefit, but I could check the BIOS/CPU-z results possibly). Oct 10, 2022 at 9:18
  • Btw, I already have an SSD :-) Oct 10, 2022 at 9:18
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    Getting a pair of ram sticks in your performance spectrum won't give you any advantage (or at most 2% more performance) over matching the specs as closely as possible without the exact model being the same
    – Irsu85
    Oct 10, 2022 at 12:29

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