The regional trains need to be expanded first: the paris transit network is star-shaped, and it's very difficult to navigate around Paris without going through the center of the star. This immediately overcrowds the existing lines, and then trains are late/you wait a lot (people fainting when it's too hot, massive queues because too many people, etc) . There's currently a massive project going on ('le grand paris', aka 'greater paris') which is supposed to achieve this. I haven't followed it very closely though.

Then, I agree with you : the trains must be reliable. However, this can't be achieved unless some kind of rerouting happens first.

It's worth noting that the speed limits apply to Paris only, which is absolutely tiny (about 100 sq km). In many (most?) cases, it's already more efficient to go to Paris by train than by car, even if not very reliable, and within Paris you have the metro. Outside of Paris, the regional trains are not very good (not dense enough), but the speed limit point doesn't apply.