wifi and ethernet slow on iMac with macOS catalina ( upgraded from mojave)

Web based speed tests depend a lot on prevaling conditions at the time of the test.

Macs may have other bandwidth limitations including:

1. Syncing with cloud accounts, like iCloud, if you store and documents on iCloud.
2. File sharing being turned on for other machines.
3. Printer sharing to share printers with other devices and computers.
4. Background backup software
5. Optimization software which should never be installed, since it actually slows you down. Use Etrecheck to identify if you have anything running in the background that could adversely affect your connection.

If your Mac has a direct ethernet port, it should be no slower than any other device on ethernet.

For true bandwidth speed, get storage space on the company providing you internet access, and do file send and receive to that space with FTP. Sending data to your internet access provider will be slower unless you have a dedicated line to them.

Often unless you have a dedicated line speeds can be very erratic, as neighborhood gamers sometimes hog the lines, and you need to report such slowness to your internet provider.

Depending on the cloud with anything less than a dedicated fiberoptic line can be futile for anything more than 50 GB.

The fact you are already in this predicament suggests you may want to see if you can spend time getting everything off iCloud onto two separate external hard drives. Once that is done, then turn off the iCloud syncing for anything beyond that which you must have on your iPad or iPhone simultaneously. Generally you want your hard drive to not exceed 85% capacity either. You are already very close to it on your Time Capsule. This causes a lot more temporary file swapping that will slow you down.