Depending on the area and time of day, the speeds can vary. LTE coverage is not nation wide and there will be some areas with only 3G. You might be getting 3G speeds because LTE might not be available in that area. Having that said, the fastest LTE networks in order are LG > KT > SK.
LG has the fastest LTE network, but coverage for all carriers is not nationwide. KT's LTE is also faster than SK, but I like SK because their calls are reliable. Most calls are still made over 3G nowadays.
SK's 3G network is the most reliable and the fastest of the 3 carriers. So if you're in an area that has no LTE, you're going to fall back onto 3G anyway. Until LG and KT have nationwide coverage for LTE, I'm going to stick with SK.
SK for all its reliability is however the most expensive of the 3 carriers. You get LESS minutes and LESS data compared to the other carriers. That's also another thing to consider. But, SK now has FREE mobile to mobile, in-network calling and SK accounts for 50% of all mobile subscribers in Korea.