I'm running into the same exact thing as you paul. I'll have to see what my polling is set to.
I'm using Cingular/Pearl and was using it with a hosted BES that worked fine. Now I'm using it with an in-house new Dell 1950 Windows 2003 server, Xeon 5050, 2GB RAM. BES Express is the only thing loaded and I'm the only user on it. Connecting to SBS2003 server.
Same exact thing as you... stuff sent from handheld is instant. It's just delivery of emails. Delays vary from 2 min to up to 10 min.