133c shows high CPU usage in the web interface even with no traffic across the link, no winbox open, etc. I’ve got 4 in testing, 3 don’t seem to be as slow, one is borderline useless. Reset to defaults and reloaded the same settings from backup and it works, but still high CPU load.
Also reboots after enabling webproxy.
RB133C is not designed to run a proxy server.
I may have misunderstood, but reading the wiki shows one way to forward the internal proxy to a parent proxy, enabling transparent proxy.
I’ll do some more reading. 
Only two things I have noticed:
If you have multiple cpus disable multiple cpu support.
If you have firewall rules (even if they shouldn’t require connection tracking) keep connection tracking enabled. Turning it off has caused me problems where routing stops even when you wouldn’t think it should.
I have been using it in production and have been pleased with its performance.
Scott