I think this could be the case when OUTGOING Traffic ist slow, but my Problem ist that INCOMMING Traffic (Access OWA behind my WAF from Outside) ist slow...
I´m facing exactly the same Problem after update to 9.307-6 OWA is very slow. DNS lookup for *.*.*.*.dnsbl.proxybl.org. failed: Temporary failure in name resolution
dnsbl.proxybl.org being down is unrelated to any UTM update.
Just enable "Skip remote lookups for clients with bad reputation" in your firewall profile (or do the cc trick from the thread linked by hallowach if you know what you're doing).