Mojave made my MacBook impossibly slow
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.