hdparm -tT /dev/whatever_your_hard_drive_is_eg_sda
Once when I had a dodgy hd on a laptop, instead of seeing 30MB/sec it was down to 120k/sec
As Nelz said, smartctl will help prove if a drive is failing.
It's been a while since IPV6 caused issues, but an outside possibiity.
Try pinging your router and google.com and seeing if you lose any/many packets.
sudo ping -w 30 -f google.com
PING google.com (22.214.171.124) 56(84) bytes of data.
--- google.com ping statistics ---
2343 packets transmitted, 2342 received, 0% packet loss, time 29993ms
rtt min/avg/max/mdev = 20.773/22.672/51.441/1.198 ms, pipe 4, ipg/ewma 12.806/22.385 ms
Average 22ms is ok.
sudo ping -w 30 -f 192.168.0.1 (whatever router/modem is)
PING 192.168.0.1 (192.168.0.1) 56(84) bytes of data.
--- 192.168.0.1 ping statistics ---
31240 packets transmitted, 31239 received, 0% packet loss, time 29999ms
rtt min/avg/max/mdev = 0.765/0.937/23.653/0.248 ms, pipe 3, ipg/ewma 0.960/0.927 ms
0.96ms is what you'd expect to see from a gigabit LAN.