Zero uses a very constrained CPU; you'd get max 100 Mbps throughput over wired, much less on WiFi. But if you only used the DNS/Adblock capabilities, it wouldn't be too bad.
I have, sort of. At my old job, just as the pandemic started, one of my coworkers didn't have vpn access and the company ran out of cisco licences before he managed to get one so I tunneled him through my pi0, afaik it was tolerable for pulling and pushing code but nothing more than that.