Originally Posted by
orinthia
Sometimes it's better not to update to the newest OS version if you're more concerned about performance than security and compatibility (or at least know how to deal with the latter two). Like my laptop from a similar time period (asus k53ta, from 2011), win10 just doesn't run as well as win7 did on it, so your mac might not be dealing with a newer mac os like it did with the version it came with. Product of the times.
Not saying downgrading is the answer, but it might be something to consider if you're not using anything that requires a newer mac os version on your old mac. I'm not familiar with what problems might arise from using older mac os versions though, if any, with apple's stuff or other modern/updated software in general. Also, knowing how to deal with the missing security patches will be key, similar to how I use my windows machines without antivirus (you really don't need it if you know what you're doing but most folks don't).
Adding an SSD where there was none can help with the symptoms though like the others said.