The fastest wait to lvl is using PSE bursts, with 50-60k xp per burst if you have a good team and get lucky with silver/gold spawns and you can chain bursts from one trial to the next if you know how it works.
IF you have premuim you can get a 10 item food buff giving 10% tri boost, make sure the xp social mag is globally full and everyone gets an xp increase (there is one for rdr and pot) as far as the XP reduction, if you have a lvl 20 main equip both lvl 15 and lvl 17 kill exp is heavily reduced and sub class is capped at 15. (the xp from the dailies are not reduced for lvl 20)
Im working on my 4th and 5th lvl 20 (18/14) and im actually taking my sweet time with it. The xp in this game thanks to pse is far too much imho.
in all fairness unless you plan to play the class you dont need to level it since if you get all the skill points (20 atm) you can spend them all in a lvl 1 and get the benefits, its not like pso2 where you had to lvl to gain the skill points to then spend them just for a sub class.
Edit to add: when the level cap goes up we will mostl likely get rank 3 maps raising the levels in that area to 30, since the xp per kill wont go up by much the 30-35 grind will take much much longer since the xp needed for those 5 levels will dwarf what you need for 19-20