There's no "correct" answer to this.
I use the Kaspersky antivirus at work for my servers and I'm happy with it. Why do you want a 3rd party firewall on a server? This is one case where the on-device job is probably well served by the built in windows tools.
Sorry, you already have SEP for what? The firewall or the AV side? As for lightweight, a dedicated virus scanner, properly tweaked, plus the windows firewall is probably as "light" as you're going to get.
I also think Dave is correct when he says that SEP ought to be up to the job. As much as I personally despise symantec, their enterprise tools aren't that bad and if performance really is bad then you either have inadequate resources, unrealistic expectations, poor configuration (e.g. are there lots of multi-level archives on this machine and the AV is set to scan them all the time?) or another unrelated issue that's the actual cause of the slowdown you're experiencing. Sorry if that sounds harsh but that's how I see it.