It's not the speed, its the acceleration. How much acceleration you have to endure depends on how fast you want to get up to speed. You could slowly accelerate up to near c and never feel more than 1 g.
That being said, the maglev around Jupiter involves a lot of acceleration. No matter how slowly you try to get up to near c, there will be a centripetal acceleration needed to keep you moving in the circular path around Jupiter that will increase as you increase your speed. You can lessent this by making the radius of the circle larger, but to bring the g force you will experience down to managable levels at 99.99% of c you need a circle with a radius of hundreds of times the radius of Pluto's orbit around the Sun.
In other words, forget about it.