If its working at all then your configured right ... there is probably an MTU mismatch, or a queue upstream from you. Again, you said things are working they are just slow, so I would think the NAT/firewall rules are working as they should.
Take a packet capture and see if you are finding lots of retransmits - also check to make sure your MTU is what it should be. Also check to make sure ICMP is not being blocked.
I have similar problem. Double checked everything.
Everything seem working correctly except
poor connection speed especially to web sides.
This issue may not related MTU.
I want to share my experience :
Hotspot user -----> Mikrotik --------->Internet
1) Hotspot(PPPoE) user that behind mikrotik router which is uses src-nat have poor connection quality either limited or unlimited BW.
2) Upload speed dramaticly 4 or 5 times faster than download speed(web connection)
3) When i tried to download something from ftp server speed
goes to near saturing internet lines. My speed access up to 1.4 Mbit. ( All same condutions)
4) Changing client machine, switch, browser etc. haven't change any result.
5) Finally another strande behaviour that when i tried flasget like program download speed going to near ftp speed result.
Its looks like something limiting per connection to particiular speed.. However i haven't set anything like PCQ.
I investigate or firewall and queue rules however i haven't found anything that related this.
So i wonder if glen66 have same problems! If he/she have
may be we found a bug!