EMET implements various protection features which are lacking from Windows editions prior to Win10. It additionally adds some protection features which catch out common cut-and-paste shellcode. EMET is being deprecated because Microsoft has now rolled most of the features into Windows 10 natively, although not all of them are present.
I wrote an answer a while back which describes some of the standard EMET features.
The performance impact is negligible, even on more complex applications like Firefox, Chrome, Office, and Visual Studio. The only issue I've ever run into is false detection of the "Caller" or "Caller+" features when opening standard dialogs (e.g. load / save) from some applications. For those cases I just turn those features off.
I have enabled EMET against pretty much everything on my system which is likely to be opening lots of untrusted content. Browsers and document readers are the obvious primary choice, video and audio players (e.g. VLC, MPC-HC) too, as well as communications apps (IRC, Telegram), plus Windows Explorer, cmd.exe, PuTTY, image editing suites, and a bunch of other random applications. These all run fine, although some needed a little policy tweaking.
Keep in mind that EMET does not protect you from malware. It implements features which make it harder for someone to successfully achieve a code execution exploit against a vulnerability in the software running on your system (not including the kernel!). These features also help stop generic shellcode from executing properly, so an attacker would likely have to tailor an exploit payload specifically for an EMET-protected system.