Yes, mixing ram can sometimes not work.
Run memtest86 or memtest86+
They boot from a usb stick and do not use windows.
You can download them here:
MemTest86 is the original self booting memory testing software for x86 and ARM computers. Supporting both BIOS and UEFI, with options to boot from USB.
www.memtest86.com
Memtest86+ is an advanced, free, open-source, stand-alone memory tester for 32- and 64-bit computers (UEFI & BIOS supported)
www.memtest.org
If you can run a full pass with NO errors, your ram should be ok.
Running several more passes will sometimes uncover an issue, but it takes more time.
Probably not worth it unless you really suspect a ram issue.
If you get NO errors, your problem lies elsewhere.
I see many complaints about gaming laptops not performing well.
Usually gaming while plugged in.
One common cause is thermal throttling.
Laptop coolers must, of necessity be small and light.
They are also relatively underpowered.
If you run an app such as HWMonitor or HWinfo, you will get the current, minimum, and maximum cpu temperatures.
For intel processors, if you see a max of 100c. it means you have throttled.
The cpu will lower it's multiplier and power draw to protect itself
until the situation reverses.
At a lower multiplier, your cpu usage may well be at 100%
What can you do?
First, see that your cooler airways are clear and that the cooler fan is spinning.
It is counter-intuitive, but, try changing the windows power profile advanced functions to a max of 90% instead of the default of 100%
You may not notice the reduced cpu performance.