Not true. Your speeds on Boost or Virgin will be the same speeds you get on Sprint. HOWEVER, keep in mind that Sprint roams on Verizon.
So, let's say you have a Sprint phone in one hand, and it's pulling down 300 kbps. Great. Lucky you.
Then, you get a Virgin phone and stand in the same spot, and it pulls down 120 kbps.
That is because even though you can't tell the difference just looking at your device, if you're on a Sprint contract, which roams on Verizon, you may have a Verizon tower near you with a stronger signal than a Sprint tower, so it connects to the Verizon tower by default. Your Virgin phone, however, will not connect to Verizon towers, only to Sprint, hence the "slower" speeds in the same spots.
So, let's say you have a Sprint phone in one hand, and it's pulling down 300 kbps. Great. Lucky you.
Then, you get a Virgin phone and stand in the same spot, and it pulls down 120 kbps.
That is because even though you can't tell the difference just looking at your device, if you're on a Sprint contract, which roams on Verizon, you may have a Verizon tower near you with a stronger signal than a Sprint tower, so it connects to the Verizon tower by default. Your Virgin phone, however, will not connect to Verizon towers, only to Sprint, hence the "slower" speeds in the same spots.