Slow System? iotop Is Your Friend
Back in 2010, Kyle Rankin did an incredible series on Linux Troubleshooting. In Part 1, he talked about troubleshooting a system struggling with a high load. At that point, I'd already been a system administrator for more than a decade, but it was the first time I'd ever heard of iotop.
If you weren't a subscriber in 2010, I highly recommend you read Kyle's entire series. Either way, I use iotop so often, I felt it was prudent to mention it again all these years later. The concept is pretty simple. It's like the top program, but instead of CPU and memory usage, it monitors disk I/O. If you have a system that is extremely slow to respond, but can't seem to figure out what is going on, give iotop a try. You'll probably have to install it, as I've never found a system with iotop installed by default, but it should be in the software repository of just about every Linux distro. And, if you find it useful? Be sure to read Kyle's entire series; it's just as helpful today as it was five years ago!
Figure 1. The Bitcoin dæmon is notorious for using a lot of disk I/O.