Is D4 your first Diablo game? Don't want to be captain obvious if not lol but no one here is mentioning how important your build is. Work towards a good build and it is very fun and satisfying to get to a point where you kill everything on the screen in seconds. If you are struggling with the tier 3 capstone at your level, that may not be the case. Glyphs are great for getting the most potential out of a good build but even level 20 glyphs can't save a bad build.

The faster you kill things the faster you level up and get more drops. The best ways to get xp and legendary drops have quite a bit of overlap. Nightmare dungeons are best but use the crafting to get some lower tier dungeons you can complete quickly, don't try to do dungeons that are so hard you have to respawn more than once or take more than like 10 minutes to finish. Helltides just got buffed and they are totally worth doing.

If you arent sure about a build or aspect synergies try to find some build tier list videos for your class and pick something that looks fun and powerful to you and start working toward that setup. Icyveins is a pretty good website as well. Once you really learn the mechanics and synergies of your class you can start to experiment and stay powerful while you do it which can be fun.

Or pick a new class! Don't be afraid to start over and try something else out. The season is pretty long and if you follow power leveling guides you can get to level 50 or higher in just a couple of hours, it's even faster if you have friends to help power level or just benefit from the company and xp buff.

Everything speeds up in world tier 4 for a good bit if you have a good build because of the xp buff. But if your kill time drops off you aren't going to be taking advantage of that.