There are a few things that could explain things!
The first is if you still have a HDD over time the files get fragmented causing the system to work harder to link up the files fragment to then access it. If you had upgraded to a SSD then you maybe facing lack of space. This causes the SSD controller to shuffle the used space around as the free space blocks are too worn. So the less free space you have the harder the controller needs to work.
In either case the system heats up! Which in turn causes the fan to kick in and slow down your system!
To solve the HDD you’ll want to make a full backup and then use a defragmenter tool to clean up the files. But you do want to clean off as much junk before you do that as the system needs free space to hold some block to then free up space for the file it’s working from. I would run Apples Disk Utility from an external booted up drive to make sure the drive is in good shape. You may want to think about getting a larger drive.
OK, SSDs are a bit different, here the current free blocks are likely well worn, so while cleaning off the junk to free up some space can buy you some time in truth it’s likely time to upgrade to a bigger drive. But you do need to be careful! As many SSDs are fixed speed SATA III (6.0Gb/s) which is to fast for your system! You either need to find fixed speed SATA II (3.0Gb/s) or get one that auto senses the systems I/O speed. So you do need to review the drives spec sheet to figure things out. If it only lists one speed that’s it nothing more (fixed speed) if you see a few speeds listed then it’s auto sense! Here’s an example Samsung 870 EVO SSD note the Interface line which lists all three data rates! FYI - This holds true with HDDs too!
In either case make sure to take the time to clean the dust build up throughout the system focused on the fan blades and the heatsink fin area. So your system can properly cool it’s self and make sure the vents aren’t blocked.
If you still have the Grey disks that came with the system one has the diagnostics see if any error pop with it.
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