How fast are your Time Machine backups?

Time Machine is intentionally relegated to a low priority so as not to overtly affect performance. Network speed is not even close to a limiting factor.

Right now, at the moment, using a fairly quiet wireless 5 GHz network, with this particular MacBook Air, on battery power, and a Time Capsule... about 300 kbps. It varies a lot depending on what I'm doing or not doing: as much as 1.5 Mbps to occasionally zero.

This screenshot shows it about halfway through a fairly large backup:

You seem to want to correlate Time Machine activity to transfer speed. It's a meaningless comparison since TM's activity is horribly nonlinear and not dependent on communications throughput including disk I/O, or whether it's over a network, or otherwise. Within reason, that is.

Time Machine was relegated to a low priority task beginning with Sierra, as part of the sweeping power reduction efforts introduced with it. With that release, any macOS processes deemed unjustifiably burdensome became mercilessly throttled or were just killed.

Needless to say, if you are using any non-Apple "anti-virus", "cleaning", or "Internet security" junk, anything at all in that broad category of useless garbage, don't. A lot of things won't work if you do. Read Sierra and Time Machine for the genesis of "TM slow" complaints that arose in Sierra. There is a lot of distracting and unhelpful noise in that particular Discussion though. This one's summation of a few key points may also be of interest.

Pathologically slow... well, I can't disagree, but whether it's cause for concern or not is premature.

I don't give TM any thought because Apple designed it to be unobtrusive. If I check the status of my backups and they're all within the last day or two I'm thrilled.

Besides the network characteristics between the Time Machine (TM) host and the backup destination device, one also need to look at the changes in the data transfer rates during a TM backup.

The image below is an 80 second snapshot at the beginning of a TM backup between a 2014 Mac mini and a Synology DS916+ NAS via a 1 Gbps Ethernet connection. The mini first performed a TCP handshake and then negotiated a SMB 3.2 connection between devices. This happens during the first 20 seconds on the graph provided below. From then on the backup proceeds to use SMB over TCP to do the actual backup. Notice anything unusual? Pretty easy to see that the data transfer rate is not at all steady. At no time where there any issues with TCP Zero Window conditions ... it's just not a very smooth process.