Mojave made my MacBook impossibly slow I have a MacBook Air 11', Early 2014 with a 1,4GHz CPU and 4Gb of RAM. I was running MacOS 10.12 without any issues, everything was fast and responsive.

Then I upgraded to MacOS 10.14, skipping MacOS 10.13. Since then, my MacBook became very slow and irresponsive. The round, colored spinning icon indicating that there is something going on is now almost omnipresent. On MacOS 10.12 I saw it rarely. Switching between apps was smooth before, now it's painful. The MacBook feels always hot, even with just the browser open. The battery burns through a lot faster.

Do I have a messed up system? If so, why didn't I have issues before upgrading to Mojave? Is there anything I can do about it?

Or is my MacBook simply too slow to run Mojave decently? Apple lists it among the devices supported by Mojave. I'm interested in the experience of other users of the same model.
I would advise using Activity Monitor to examine the CPU usage of the running processes. There's most likely a process taking up huge amounts of CPU-time responsible for slowing down the rest of the system.

When you have identified the process, decide whether to remove the program, look for updates, contacting the developer or otherwise rectify the issue.

Note: Right after installing Mojave the system will be slower than usually because it is creating Spotlight indexes, caches, and other types of one-off computations. Please wait a day or two for this to complete.