Just wondering if focusing with this lens is still slow on a 1d series body?

All lenses will focus faster on 1D series.
However, the tammy 28-75 will still be a bit slow.

It also depends on what you shoot.

Fraga is correct. My Tammy hunts on my 1D just as much as it does on the 5D II. But if you're not using it for sports or action, then you'll likely enjoy the use of this lens.

You won't see much of a difference. It's one of the things that sets the Canon L apart - virtually instant focus in at least decent light. Whether it's worth the extra bucks is up to you.

I owned 28-75 for 1 week and tested it on 5D. Its AF is actually pretty fast and I think it's similar to my former canon 50mm 1.4. On top of it, it's pretty accurate too.
Of course when you pair it with 1 series, it'll be even faster.