Sprint does have the worst 3G speeds of the four major carriers. Of course, YMMV, so much with cell networks depends on coverage in your city, particularly the areas where you are using your phone.
There seems to be evidence that Virgin Mobile is even worse, as it appears that Sprint gives the traffic from their MVNOs a lower priority. I'm not quite sure how it is done; I've seen some claim it is because Sprint limits the towers that Virgin Mobile customers can use and other claims that say they give a lower priority to Virgin Mobile users. There appear to be no admissions by Sprint or Virgin Mobile that this occurs; the sole evidence is largely based on speed tests done in the same locations on both Virgin Mobile and Sprint phones. It will be interesting to see how this plays out on LTE.
GSM carriers (AT&T and T-Mobile) have the fastest "3G" speeds because of the technology. The US CDMA carriers (Verizon and Sprint) never expanded their 3G standards; instead Sprint quickly moved to Wimax and Verizon became an early adopter of LTE. GSM 3G standards were improved using HSPA+, which you could call 3.5G. AT&T has maxed out their network at HSPA+ 21 (theoretical maximum speed of 21 Mbps) and T-Mobile has gone up to HSPA+42 (theoretical maximum of 42 Mbps) -- in the real world, maximum speeds are about half of the theoretical maximum. So, in terms of 3G, T-Mobile has the fastest 3G data network (to the point they marketed it as "4G", since their network download speeds are similar to what most LTE users are getting) but, like Sprint, they are a smaller network (compared to AT&T and Verizon), located mostly in population centers and with poor coverage in rural areas.
I quit Virgin Mobile a little over a year ago because I just couldn't take the data speeds any longer -- and Wimax was too spotty in the areas I use my phone. With T-Mobile I am getting download speeds (on an HSPA+ 21 phone) that range between 3-8 Mbps. It has been a huge improvement for me and I have solid coverage in most areas. If I had an HSPA+42 phone, my data speeds likely (based on results in my area I've seen) would be a bit more than twice as fast (so between 7 and 17 Mbps). Again, your experience will depend on coverage in your area.
Last, one good tool that might give you an idea of the speeds between the various providers is
PC Mag's Fastest Mobile Networks article. Just remember that many of the cities the tested that do not show T-Mobile and/or Sprint having LTE do now have LTE coverage (or likely will by the end of the year).
