Quick Tips:
Wi-Fi
Disconnect from your wifi. Even if you have the ethernet cable plugged in, often your computer will favour using wifi.
DON'T USE WIFI
Laptops
Plug in to a power source. If you run off battery, the laptop may use a low-power mode and be slower.
If you need to run on battery, then use the Power Options on the computer to run in High Performance mode.
General Internet:
Connection will be worse if other people in your house are using the internet at the same time, especially for things like youtube/netflix. Try to play when family are not watching netflix/youtube.
Go to this site (https://speed.cloudflare.com/) and observe the "latency" and "jitter" measures. Latency corresponds to lag(delay). Jitter corresponds to lag-spikes, which is the worst thing.
Ideally your jitter should be very low, like under 5ms.
Don't say "I've got a 100Gbps connection!" and think your connection is great. Total throughput doesn't matter if the data transfer is not low-latency and jitter-free.
Anti-Virus
Uninstall/Disable Norton anti-virus/firewall. It goes overboard and slows down the data packets coming in to the emulator, causing lag. "Windows Defender" is fine and free (build in to windows). If you can figure out how to add an exception in Norton so it doesn't affect your RetroArch executable, you can do that instead (and you'll need to do it every time you update RetroArch to a new version). Same with McAfee, too... you know it's not what you need when they try so hard to give it away like an extended warranty on a toaster.
Frame Rate Counter
If your framerate strays much from 60fps (frames-per-second), you or your opponent could get more lag-spikes.
To toggle framerate display, press the F3 key (or Shift+f3 or Fn+F3 depending on your keyboard)
To get a better(accurate) framerate count: Go to Settings->On Screen Display->On Screen Notifications and change the FPS framecount from 256 to something like 60
Video Filter
In my experience, the RA "Video Filter"s are SLOW, so don't use them. If you have accidentally selected one, use the Start button or Space key to clear the filter.
Laptops
This is my opinion.. it's your money, do what you want.
Cheap laptops are garbage, from day 1. Suggestions:
Don't buy anything under US$450 / CA$600 new. (If you really must get something cheaper, get it Used but also obeying the following specs)
Make sure it has at least 8GB of RAM. If no video card, try to get "dual channel" RAM (e.g. "2x4GB" is 8GB of dual-channel RAM), to avoid RAM access conflict between the CPU and integrated graphics.
As a rule of thumb, personally I would only consider Intel i3/i5/i7 CPUs. No Celeron, Athlon, Pentium Gold, A10, Atom.... Removed from recommendation: AMD Ryzen (top-of-the-line chip but apparently doesn't handle RetroArch well.. NOTE: Getting dual-channel RAM may fix this?).
The hard drive should be a SSD, not HDD.
Screen resolution: at least 1920x1080 (Full HD, FHD) unless you're getting a really small screen (<=13").
NO CHROMEBOOKS. It's not a "real" computer for running software. It's a 300$ web browser.