I like nova also.
Lag?....That's hard to answer. Lag depends on your device and what running services are present, and your perceptions. I can say nova is the most used 3rd party launcher today...probably indicates it causes no problems on MOST devices but again mileage varies by specific phone etc. It is known for its features and ease of use but not necessarily for being "light" (low memory footprint). Foot print and resource usage also depends on what you put on home particularly widgets. There are other 3rd party launchers that tout themselves as light launchers well suited for low phones.
I have an s4...don't notice any lag problems with nova. Do notice general lag occasionally which starts several days after reboot. . I set a reminder to reboot every day at a convenient time vand that keeps me running smooth.
nova had setting for aggressive desktop which presumably would decrease any lag for launcher but might claim more higher priority memory and make other apps more likely to lag (again depending on those factors).
I've never used go, but if you search around, you'll see Go launcher has a bad reputation. As I recall concerns with privacy and with "bloat" and maybe ads. Even if you don't choose nova, I'd avoid go.