Mac mini experiencing slow network speeds on iPV4

I concur with MrHoffman, this issue appears to be on your local network.

Let's check Ethernet first ... as such, temporarily disable your Mac's Wi-Fi connection so we will only be testing the Ethernet one between your Mac and your Asus router.

Checking the Mac side first, go to System Settings > Network > Ethernet. For each setting on this pane:

• IPv4 Configured: I'm assuming that it is set for: Using DHCP ... correct?
• IP address: This would be the local IP address assigned by the DHCP service from the Asus. Does it look correct to you?
• Subnet mask: Should be 255.255.255.0
• Router: Should be the LAN-side IP address of your Asus.
• DNS Servers: There should be a pair of DNS servers listed. Depending on how you have your network configured, these could be the ones that your ISP is providing you, or ones you specifically set up manually. Regardless, do they look correct to you?

For now, we can ignore the Seach Domains & IPv6 address settings.

Now select the Details... button. The first window would be for the "Limit IP address tracking" setting. This setting may be grayed out. Regardless, we can ignore this one as well for now.

Select the TCP/IP tab. Since we are only concerned about IPv4 for now, you can optionally reconfigure the Configure IPv6 setting to Link-Local Only which would effectively disable IPv6 traffic coming/going from/to the Internet via the Asus.

Select the DNS tab. Again, you should be familiar with the DNS IP addresses listed here. If they are grayed out, that means that they are being assigned to your Mac from the Asus.

Select the 802.1X tab. Unless your network requires the use of an user authentication server, then the "Enable automatic connection" setting should be disabled.

Select the Proxies tab. Unless your network requires the use of proxies, all of the settings should be disabled.

Select the Hardware tab. Unless you need to use specific settings, like the use of Jumbo frames, the Configure option should be set to: Automatically. Now check the Speed, Duplex, and MTU settings. Are they what you expect for your network configuration. For example, for a 1 Gbps connection between your Mac and the router, Speed should be set to: 1000baseT. The AVB/EAV mode setting can be left enabled.

Now with these settings checked, we are going to do some simple ping tests between your Mac and the Asus, starting at the Mac end.

• Ping Loopback Address to verify that TCP/IP is functioning on the Mac's Ethernet interface:
  • ping -c3 127.0.0.1
• Ping Localhost to test the host name cache resolution:
  • ping -c3 localhost
• Ping the Mac's IPv4 Address to test the Mac's IPv4 address:
  • Use ipconfig getifaddr en0 to display the Mac's IP address assigned to the Ethernet interface (en0)
  • ping -c3 <Mac's IP address>
• Ping the Mac's Host Name:
  • Use hostname to display the Mac's host name.
  • ping -c3 <hostname>
• Ping the default gateway (your Asus)
  • ping -c3 <LAN-side IP address of the Asus>

Note: All of these pings should result in three packets being sent & received without any error and with very little latency.