I know this is an old thread, but for the folks saying don't vacuum the iMac's vents: I just did it, and it seemed to help substantially. I have a 2020 iMac 27" with an i9 and 5700XT, so with the combination, it's definitely a bit thermally-challenged.
How I Cleaned My iMac
First, I unplugged the computer and placed it face-down on a flattened cardboard box. I took a toothpick and very gently ran it along the dust filter at the base of the iMac's chin. The intake holes that are machined into the aluminum chassis actually have empty space behind and between, so it's kind of like "flossing" out the bigger chunks. Then, I used a Dyson v11 cordless with a hard tipped crevice attachment, and after some light passes over the exhaust outlet at the back of the computer and over the intake vents in the chin, I managed to fill up about 1/5 of a canister with about 3 years' worth of dust.
How It Behaved Before
I noticed the machine would hit 100C sometimes in Intel Power Gadget just loading websites in Chrome. If I play Minecraft with any shaders that have basic water reflections, it would drop to 1FPS within 2-5 minutes, and in Activity Monitor, "kernel_task" would spike to use over 1,000% CPU (basically consuming all available cores). Some forum posts claim that's Apple's software management method to do last-resort thermal throttling by resource-starving other tasks.
How it Behaves Now
With all the dust out of my machine, it now runs at about 70-95C, in the extremes of either idling or running Minecraft with shaders, respectively. And "kernel_task" has thankfully not had to kick in, which was my main gripe as it made the system unusable (by design, supposedly).
Again, I think if you're careful about this and use an extension with a hard plastic tip and only hover the suction above the vent holes, the risk isn't too bad to damage the machine. If you're using a powerful corded vacuum on carpeted floors, you may have higher risk. It seems the 2020 iMac, with it's 10-year-old chassis and thermal design, isn't too well-suited for some of the higher end options in the final year before Apple discontinued it. The sensor and software calibration is very sensitive to any spikes beyond 100C at the CPU cores, and I have a feeling it was tuned specifically for a near-obstruction-free scenario with full airflow.
I think periodically cleaning it every year or so, depending on environment, is going to be a requirement to retain performance. I would even so so far as to say that although initial reviews of the 2020 iMac say it outperforms an iMac Pro, those were short-term reviews where dust probably wasn't a factor, since the Pro did have a better thermal design.