Tor is already slow - people use it because they want security. The addition of another VPN increases security greatly and only adds a minimal amount of more latency.
Relatively, sure. But I've found it very usable in recent times actually. Used it almost full-time (besides a normal Firefox instance for the company's intranet) to get around some silly firewall that wouldn't let me download "hack tools" (I was an intern in the cyber security department, security tools were part of my job). There were times where I didn't notice at all that I was using Tor, and most of the time it was comparable to mediocre wifi.
You would usually not try to block Tor at the network level. You would lock down your computers so employees can't make changes, and only allow them to run executables from locations which they have no write access to.
Tor can and should circumvent any firewall using obfuscation proxies that use AWS, GCS, Azure, etc. You'd need to block most of the internet to kill Tor.
And as for monitoring, I guess it might be possible, but if someone thinks to use bridge nodes that's also defeated.
