There are a few reasons:

• most software eventually gets slower for all computers; new computers can hide this because they're faster
• websites use more resources nowadays than they used to; new computers can hide this because they have more resources to give

For things that you can control, your operating system can become bogged down over the years with detritus, so reinstalling your OS can speed them up somewhat. Just make sure you back up your files and that you have the installers for the software you want to use.

If you can't reinstall your OS because you don't have the OS installer, you can install some free but relatively easy-to-use OS for older computers like Xubuntu 18.04 32-bit (make sure there's an X on the front of the name). The OS installer has a try-before-you-actually-install-it mode where you can connect to the Internet and see if Firefox works and YouTube videos play with sound and try out other software you care about, like LibreOffice for word processing and spreadsheets. Takes a little getting used to, but it'll look pretty familiar if you've used Windows XP.