Booting up in safe mode isn't actually a fix for the problem. It is just a diagnostic tool. Safe mode starts the computer in a basic state with a minimal amount of essentials-only running on the PC. If the computer runs OK in safe mode, then the basic functions of the computer are running OK. It is just a means of troubleshooting the PC. Once you have finished troubleshooting you would put the PC back to start up in normal mode.
When you have started the computer in safe mode, does it start up normally and run at a normal kind of speed?
If so, the next stage would be to look at what you have running on the PC, when it runs normally, which might be causing it to slow down.
If you are not that familiar with how to diagnose and troubleshoot PC problems, you would probably do best to find a local computer tech or computer shop to help you out.
