Run Activity Monitor to see what's using resources. Something is probably hogging CPU. Also, try EtreCheck. It's a good, free diagnostic tool. I also use iStat Menus, which tells me at a glance what's going on in this kind of situation. It could be as simple as something like the (totally redundant) Samsung Printer Monitor, that a colleague has, which is forever shitting itself and using 100% processor. Fans run up, machine slows down...
These days, kernel panics most often mean hardware. You can use MemTest (or one of many non-free tools) to check your memory. Various utilities are available to test your HDs.
If it's something easily replaceable, like memory or an HD, then I would stick with what you have. Newer machines are improved in terms of power consumption and graphics performance, but that processor is still very powerful.
You might as well do all the usual software things too though - create another user account and see if the behaviour is any different. If yes, then I would reinstall everything from scratch. It's always good to find out what's wrong, but it's very time consuming. Nuking from low orbit is much quicker and it will almost certainly speed things up anyway, especially if you haven't done it before or for a while.
I would definitely put a big SSD in it if the Fusion Drive is causing problems though, and maybe even if it isn't but you do identify and rectify the fault. They're pretty cheap these days and they make a massive difference to everything.