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I currently have a laptop that is about two years old, and has 8 GB of RAM. I use my laptop for internet browsing, productivity applications, and programming, including data science within Spyder or RStudio. I'm running Windows 10.

I usually sit around 65% memory usage, or 5 GB roughly.

I recently purchased, but haven't yet received, a laptop which has 4 GB of standard RAM, and 16 GB of Intel Optane memory. The laptop was marketed as having 20 GB of RAM, but after reading more about Optane memory, it seems that it serves more as a low-latency cache than as RAM.

For the purpose of running several memory-intensive programs simultaneously (e.g. browser with many tabs, data science IDEs) which average 5 GB of RAM usage, will decreasing RAM from 8 GB to 4 GB but adding 16 GB of Optane memory cause a performance slowdown?

Here's a snapshot of my current memory profile if it's helpful:

Update: if you're curious, I was able to cancel the order without issue. The answers helped me realize that the new purchase would have most likely been slower than my current laptop.

Intel Optane "memory" is a misnomer. It is just a cache memory card that can turn
a simple hard disk into a hybrid disk, by adding a RAM cache that can work at the
speed of an SSD disk.

Optane memory isn’t a random-access computer memory, or RAM.
Instead, it's meant to work as a cache memory bridge between RAM and storage,
allowing for faster data transfer between the memory, storage, and processor.
As such, it's magnitudes faster than a spinning hard drive and can work at the
speed of a budget SSD. And like SSD, it doesn’t erase itself when powered off.

Conclusion: You did make a mistake by falling for a commercial hype.
In addition, 4 GB of RAM is ridiculously low in our current state of hardware
and software. I would advise canceling the order for this laptop,
if you still can.

The other answers cover the gist of the situation but I'd like to add some context.

For a long time, RAM has been often called 'memory' in laptops targeted at the layperson. This is usually helpful and hasn't caused issues, until recently. The same companies that started this practice are now lumping Optane cache and RAM under the same name 'memory' with the intention of tricking you into thinking that the laptop has more RAM than it actually has. Optane technology is still relatively new and unknown so this deception is very easy to fall for. Looking deeper into a product listing will usually reveal the truth.

Optane cache doesn't, and wasn't meant to, replace RAM. They both help speed up a computer, but operate differently and in different situations. Having Optane won't alleviate the heavy slowdown of only having 4GB of RAM when normal use requires at least 5GB.

You've fallen victim to intentionally deceptive marketing. I recommend purchasing a different laptop (with 8GB or more of RAM) to satisfy your use case. Perhaps one from a more upfront manufacturer.

Only 4 GB? I hate to break it to you, but to the full extent of my knowledge (which isn't much) Intel Optane is not like memory aka RAM; it's like an SSD that takes the applications you use most and puts them on a faster drive so they load faster.

So an Intel Optane drive might eliminate the need for an SSD (please feel free to correct me on this). Your manufacturer probably put an Optane drive in your laptop and marketed it as RAM because 20 GB of Intel Optane is cheaper than 20 GB of RAM*, but maybe you can manually change the RAM to 8 GB for about $80.

*24 GB of laptop RAM $182
24 GB of laptop RAM

4 GB of laptop RAM and 16 GB of Intel Optane memory $87
laptop RAM
Intel Optane memory

To be pedantic, a Windows 10 system will run okay on 4 GB of system RAM. Between paging (with Optane, would be less painful than plain HDD) and RAM compression, you have somewhere between 6-8 GB of RAM you can use before you'd notice any slowdown at all, and Optane can cache several gigabytes of commonly used files to improve swapping performance. It's possible that you'd not notice any loss of performance if you're simply writing code all day.

You'd still notice some obvious slowdown compared to a system with 8GB of system RAM, but not as much as you'd think, so long as you're not doing anything too memory intensive. I have 12 GB in my system, and I rarely hit even 7 GB used, even with 2GB for chrome and 1GB+ for my development tools. If all you're doing is development, 4GB should actually be plenty for your use case, but if you plan on playing games or editing video, then you'd definitely need an upgrade.