1. Hard drive is almost full or fragmented
Files that are needed for programs to operate are stored in the hard drive, so as storage space fills up, it may cause your computer to operate at a slower pace. You may notice performance issues long before your hard drive is full.
When files are written over, it may be fragmented and exist in multiple places, so the processor must retrieve them from many sections of the hard drive. If opening a file takes a long time, disk defragmentation may help.
2. Not enough memory
Depending on how long you have had your computer and how you are using it, you may need to increase your memory (RAM). In general, 8GB is sufficient for most users, but again this may vary for different use cases.
3. Processor is overheating
Your PC is going to heat up a little as your processor perform heavier tasks. That is why your computer has a fan - to cool down the processor.
If your processor is overheating, check to make sure the fan is not obstructed, that it is clean and free of debris, and that it is functioning properly.