SSD upgrade is the best bet for any system still on platter drives. The hard drive is typically the bottleneck in most older systems. Now that SSD prices are dropping, it has become a relatively cheap upgrade too. You’ll notice a significant improvement in boot times, wake up times, and program launch. It is a good moment to re-install everything too, starting with the OS. Save all your important files and start fresh, as your registry is probably a mess by now. The rest of your computer set up will feel like a new machine after the SSD install. Only caveat is that this still won’t help increase frame rates for gaming all that much (or at all). It definitely won’t hurt, but it won’t be a miracle upgrade in that front. This is just a general productivity and ease of use improvement.
I wish there were an addendum to this article that said, “Give up, you’ll save money (via electricity usage) and time by just replacing it with a Raspberry PI if your hardware specs are lower than: ... ... .”
For phones I have a simple tip. Delete the Facebook app. The actual app from the Facebook company is a resource hogging monster. It has been noted to use upwards of 2 GB of RAM by itself. If you aren’t using a high end phone you should use one of many lighter weight Facebook clients.