The first point is rarely a problem, modern desktop and laptop computers are usually powerful enough. The most important aspect is a relatively fast CPU, and especially how fast a single thread can run.
The second point is more tricky. Power saving features and other resourcy-hungry applications can interfere with the emulation. Sometimes to the point of the emulation not running fast enough, but more often the result is stuttering and un-even emulation speed.
In order to get smooth video output with stuttering (and without audio issues), the emulator must be to both emulate frames and display then on screen very quickly (typically 50 or 60 times per second). Modern games running on your system can typically account for small delays and uneven performance, but old computer systems ran at a fixed speed. So even quite small delays in generating and displaying a a video frame can cause noticable video stuttering (or crackling audio).
Write about the performance overlay (Mod+O).
On many systems, the application will get less run-time when it's not running in the foreground (meaning not having mouse and keyboard focus). So if you notice un-even performance, make sure that the emulator window is focused.
Running in full-screen might further ensure that the emulator gets enough resources.
Avoid powersaving, this can throttle down your CPU/GPU making them run much slower than they can. Powersaving can also cause the emulator getting too little run-time, for example by pausing the emulator slightly, but long enough to cause stuttering.
If you have Linux; install the gamemoded daemon to allow the emulator to automatically switch to the performance CPU governor while the emulation is running.
Depending on your system, your computer may be throttled when running on battery alone. Switching to a performance power profile might alleviate this, or maybe not. You can try running with or without external power and see if it makes a difference.
Especially web browsers can use more CPU and resources than you might think, depending on the open tabs. On the other hand, if the applications are idle, they should not interfere.
If you believe other applications might interfere, you can try closing them and see if it helps. You can also use the task monitor of your operating system to see what applications are using resources.
You might recall running UAE on an old 200 Mhz Pentium processor, and wonder how it is possible that performance is even a topic in 2020! The emulation accuracy in modern UAE emulators is much better than it was 10-15 years ago, and this increased accuracy comes at the cost of requiring more powerful computers.
Emulating the Amiga 1200 is especially demanding, since the CPU is clocked twice as high as the CPU Amiga 500, and the emulator spends most of its time emulating the CPU.
Don’t emulate an Amiga 1200 if an Amiga 500 or an Amiga 600 will do. The CPU in Amiga 1200 is twice as fast as the one in Amiga 1000/500/500+/600, and you need a much more powerful computer to emulate A1200 at highest accuracy.
For example, many people use Amiga 1200 settings when running WHDLoad-installed games. But for most games, an Amiga 600 with Kickstart 2.0 will work equally well (or better), but will require a lot less resources to emulate.
FS-UAE requires more CPU and GPU performance when certain options are enabled. For example GPU performance will be affected when using pixelshaders.
FS-UAE will usually have higher performance with video sync turned off.
FS-UAE can use less color depth in the internal video buffers as well as in the textures uploaded to the video card. For slower/older computers, this can give a nice speed boost (but should not matter for modern PCs):
video_format = rgb565
(This option will also automatically set texture_format = rgb5).