Our desktop computer was starting to get pretty sluggish, so a couple of weeks ago I did a fresh install of the OS (Ubuntu 18.04). Things were better for a few days, but that hasn't lasted.
Performance seems to get gradually worse throughout the day. It is almost entirely used for internet browsing, streaming TV and general office use, so nothing I would consider too intensive.

The computer is about 5 years old, so it's certainly not new, but I was hoping to get a few more years out of it at least. Specs are:

Intel Core i3 6100 Processor
4GB DDR4 2133Mhz Memory
500GB SATA Hard Drive
Intel H110M Chipset Motherboard

Any advice or suggestions greatly appreciated.