Q: How can make my 5 year old Mac faster?

IF your device is actually limited by RAM (check how many pageouts you actually have in VM - forget about RAM used or VM size, but look at actual pageouts), then adding RAM may help a fair bit.  However, if it has plenty of RAM for the use you normally put it to, then adding more RAM will not do anything at all.  So, unless you are seeing many (1000's) of pageouts, adding RAM is unlikely to make a noticeable difference.

If you routinely work with large files, so have a lot of disk activity, swapping your hard drive for a solid state drive may help too.  However, if you do not normally do anything disk intensive, then the expense of this upgrade may well not be worthwhile, as you'll gain little in daily use performance.

Otherwise, there really is nothing you can do - the cpu, logic board and bus are not upgradeable, and they are the ultimate limitations of performance on any computer.

Check with your ISP.  How much speed are you suppose to get from them?  Check your own speed by using the Speedtest.net website.

How large is your hard drive and how much hard drive space do you have left?  Little storage space will slow down your computer and make it do funky things.

Run "repair disk" from your install or system discs.  It's located in the Utility menu.

RAM, assuming you are doing any serious paging.

Replace the disk with a solid state disk.