Lets not lay blame to any unrelated parties. k?
My guess is simple cause .. hum, Look at the services?! Yahoo and Gmail are GIANT companies with MASSIVE amount of hardware and an insane architecture behind it. DH is just a webhost that would rather sacrifice some speed for security. SquirrelMail (SM) is horrible. I refuse to use it, ever, as my main email client. I just forward everything to Gmail. I'm thinking SM's a bit slow due to access cross mounted mail directories for your email. I see SM as just a convenience, so I've never concerned myself on any SM issues.
/nod, that was my main email client of choice, until I switched to Gmail.