Have you tried using another browser such as Firefox, Vivaldi, or Google Chrome? Safari on macOS 10.13 has not been updated in several years so it is out of date and may not be able to load some websites properly.
Perhaps you are using a browser extension that is causing a problem. Or perhaps some other third party software is interfering with the normal operation of macOS (anti-virus, cleaning apps, third party security software are notorious for causing problems). To look for possible software issues run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.
Using Safe Mode and creating & using another user account can help you narrow down the problem as well.
Try running the Apple Diagnostics to see if any hardware issues are detected.
It is also possible the hard drive is worn out or failing, or maybe there is a file system issue. To check the file system run Disk Utility First Aid on the hidden Container. You may need to click "View" within Disk Utility and select "Show All Devices" so that the hidden Container appears on the left pane of Disk Utility. Even if First Aid shows everything as "Ok" click "Show Details" and manually scroll back through the report to see if any unfixed errors are listed. If there are any unfixed errors, then try running First Aid from Recovery Mode and if that does not fix those errors, then you will need to erase the whole physical drive before restoring from a backup.
Your laptop should be able to run macOS 10.13 without any problems although macOS 10.13 has not received any security updates for a few years. If you are familiar with Linux, then Linux would be a good option for having an up to date OS in order to extend the useful life of the older computers.
