Mac OS high Sierra 10.13.6 2010 21.5 inch

a) Install a less demanding operating system, for example Linux (Mint, Fedora, etc).

b) Install more RAM if you still have the factory default 4GB. This doesn't improve system startup or file opening speed, but once everything is open it should run much faster. I'd suggest 8GB minimum with an SSD or 12GB minimum with an HDD.

c) Replace the internally HDD with an SSD. This will significantly improve system startup and file opening speeds, and the faster swap space will make RAM less critical. A 3rd party thermal sensor cable is required, such as those sold as part of upgrade kits by OWC. OWC SSD Upgrade Kits For 21.5-Inch iMac 2010 (macsales.com)

> Caution: internal drive upgrades are moderately difficult DIY projects that almost need a 3rd hand. Consider having a second person around for assistance, using an iMac service wedge, or paying a computer repair shop to perform the upgrade for you.

> Reminder: make both a backup and a bootable USB installer before starting. Create a bootable installer for macOS - Apple Support

Note: I'm assuming speed is the only real issue. If you are also having software or website compatibility issues then I would suggest not investing money into component upgrades and instead invest in a newer computer.

Those Macs don't support USB 3.0, so a USB 3.0 / SATA SSD would be bottlenecked to USB 2.0 speeds. Even FireWire 800 will bottleneck a SSD, so you wouldn't get blazing speed – only steady performance.

It may be that the Mac is simply too old for what you expect of it.

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support