As others have mentioned, there are two main differences: effect and duration.
I’m going to assume that street speed = amphetamine sulfate (aka racemic amphetamine). The problem with ”street speed” is of course that you won’t always get what you hoped for.
The difference in effect is because Vyvanse (lisdexamphetamine) is converted to dexamphetamine only, while street speed (racemic amphetamine) releases a mix of 50% dextroamphetamine and 50% levoamphetamine.
Dexamphetamine releases significant amounts of both dopamine and norepinephrine and is much stronger than levoamphetamine. Since it releases a lot more dopamine than levoamphetamine, dexamphetamine tends to produce more mental effects (focus, motivation, euphoria) and less physical effects.
Levoamphetamine is weaker and releases much more norepinephrine than dopamine. Because of this, you get less of the pleasant mental effects and more physical effects (increased heart rate, jitteryness, feeling tense and wired). However, you still get some mental effects from norepinephrine (like alertness).
The other difference is that Vyvanse’s active substance (lisdexamfetamine, which is dextroamphetamine linked to lysine), becomes a prodrug that is gradually converted to active dexamphetamine, prolonging the effect. Street speed (racemic amphetamine), on the other hand, is already active and does not need to be gradually converted first, so its effects tend to come on faster and last for a shorter duration.
Because of these differences in effect and duration, street speed (with its 50/50 mixture) will often cause a stronger initial rush. The main reason being the lack of gradual conversion. Another contributing factor is that it contains levoamphetamine, which tends to add more alertness and physical stimulation.
After its initial push it will probably feel like a more alert and physically stimulated Vyvanse experience until it wears off. When it wears off, it’s not uncommon to experience a more intense crash than from Vyvanse. Since levoamphetamine unfortunately has a longer half life than dexamphetamine, you might remain physically amped but with less of the pleasant dopaminergic effects from the dexamphetamine. So once the good times have faded, it’s a good idea to prepare to remain anxious, jittery and unable to sleep for quite a while.
I should also mention that these differences in effects are highly subjective. It’s not uncommon for people to feel calm on racemic amphetamine while being an anxious mess on Vyvanse, for example.
Finally, if by "street speed" you actually meant meth, there are some additional differences, such as serotonin release and an increased ability to cross the blood–brain barrier, resulting in higher potency and a slightly different, more serotonergic feel.
