your exp is based on how much higher level the stuff is that you kill, how much link to it, your exp chain, your max tnl.
the exp your level 50 has the potential is the highest in the game.
the max amount of exp you can get credit for scales with your level, IE a level 10 can only get higher bonus on mobs like 3 levels higher than it, it gets no benefit for killing a mob 5 levels higher
it all comes together to basically, the exp you are getting at 50 is not the exp people will get while leveling up. There are not many camps outside of strongholds that can support that mass murder, and the base tnl is way lower so your base exp potential when multiplied is way lower (caps at 100% from links and exp i believe)
essentially they smoothed teh curve out, you spend longer time at lower levels, and a little bit less at higher levels(than before 1.19, you of course still burn through level 1-15 pretty fast, and it will take you longer to get 40-41 than 30-31), which all in all i think makes more sense from a game design standpoint.
while i enjoyed aspects of the old system, i think the exp chain and link style is more gratifying overall, and makes better use of the game design from battle. One target fighting made very little sense with most jobs skill sets.
pling and overall level speed is a different issue, and your progression right now is tied more to your materia hunting/creating. They simply cant make a game that caters in amount of grind to the top 5% of people who play. I can see wanting advancement and content at high levels, but if they based the amount of time required to reach level 50 on your personal preferences, not many people would be satisfied.
get materia x5, it takes forver. you know how much grinding you have to do to get a good teir IV for your class? then how many you need to blow up? and its actually worth it stat wise? thats your progress after cap right there.