Ever since I installed iOS v4 on my iPhone 3G, my phone has been running extremely slow. Simple applications like Camera, Safari, Text Messages, and others were simply crashing. Other apps took about 45 seconds to 1 minute to launch. Typing was horrendously slow as each letter took 5-6 seconds to appear. The actual upgrade itself to iOS v4 took 5.5 hours, as you may have heard in my Twitter stream. Ugh!
I tried tips that were mentioned in the blogosphere, but nothing. 
But then I got a fix! Yesterday, I got a note in my inbox how some users were experiencing problems while syncing their Mail, Contacts and/or Calendar after upgrading to iOS v4. While I wasn't having any specific issues syncing my Mail, I decided to take a peek at the technote published by Apple. The 'Symptoms' section had something very interesting:
Immediately after updating to iOS 4, some users may notice that Exchange ActiveSync Mail, Contacts, or Calendars do not sync, or sync very slowly.
The part about "very slowly" caught my attention and I decided to install the patch. I then installed the patch in less than 1 minute, restarted my iPhone 3G and voilá! Now I have an iPhone 3G with iOS v4 running very quickly (as quickly as it can run).
Therefore, if you are experiencing slowness in your iPhone 3G with iOS v4, I recommend that you install this fix: http://support.apple.com/kb/TS3398
YMMV.