The theoretical limit is down to the integration time for the receiver,
typically 1-10ms. If you want high dynamics, you need to update faster.
For car racing, it's probably closer to 1ms.
Most comercial receivers output 1 position per second because it takes
some number crunching to compute the satellite positions and solve for
the receiver position. This is purely a matter of computing power.
Given a fast enough processor, 1000 positions per second is possible.
I was running an automotive receiver IC (60MHz,32bit MIPS processor)
at about 10 fixes per second, getting close to 100% CPU.