IRCTC website is hosted on the server and the server speed is not measured in terms of 3G and 4G. That speed is measured in terms of the bandwidth. And the server's bandwidth, number of concurrent connections decide the speed of the website. And that decides how the performance of the site comes into the play.
So the consumer having the 2G 3G, 4G or even the future technology doesn't make sense to the IRCTC server. It has to increase the speed from it's own servers. And they have to increase the capacity from their place onwards. If the IRCTC doesn't have the CDN, bandwidth etc then the speed will continue to go low. And they have to upgrade on the infrastructure.