You can use 'Activity Monitor'
( menubar Go-->Utilities-->Activity Monitor )
It will tell you which programs are soaking up resources.
I suspect it will be related to lack of memory (RAM) rather than anything else. Most Mac models from 9 years ago were upgradable by adding more memory, though many today are not.
I'd say a minimum is 4GB, or pfeferably 8GB, if you can get that.
What model is it?
You might also want to stop some programs loading at startup
Settings-->users and groups-->yourname then click the 'login items' tab and use the + and - options to remove any that you no longer need/use.
I'd also clear out the hard drive, if you can. 80% full is getting a bit high for a mechanical disk. Maybe get an external SSD drive or large capacity USB stick and put some or all of the files on there either temporarily or permanently.
Good luck!