There are a lot of things you can do to maximize your XP/hr. Some of them are very simple, others are a bit more complicated and require more skill/planning. I might come back and add some more complicated/advanced strategies to maximize xp/hr, but for now here's a pretty basic list that should get most people started.

Basic Strategies

• Run 3-10 grift levels lower than the max level you can run.
  • Depending on your gear, class, and whether or not you're solo/in a group, this number will vary:
  • If you're a naturally tanky solo class (crusader, barbarian, certain builds that are tanky) you can run closer to your max gr level and achieve very high speed rift levels.
  • If you're using pools of reflection for the xp bonus, this number will decrease (because if you die at higher speed rift levels you lose the 25% bonus from the pools AND the time it took to gather those pools).
  • Most importantly, do what's most fun/comfortable to you. Switching it up will probably increase your xp/gains because you won't be as bored playing the same thing over and over again.
• Gather pools of reflection if you're over paragon 1000.
  • You can gather them at lower paragon levels (I suggest not ever gathering them below paragon ~800) but it's a decrease in terms of xp efficiency because pools of reflection only stack 10 times based on your current xp-to-next-level*0.4 and your xp-to-next-level increases with your current paragon level.
  • Here's a list of areas that have a high chance to spawn pools.
  • The cow level always has 2 pools of reflection. You can open the cow level with Bovine Bardiches + Kanai's Cube. Craft Bovine Bardiches from rare polearms using the rare -> legendary Kanai's Cube recipe. Make sure you're on a level 70 Crusader to maximize the chance of upgrading to a Bovine Bardiche (monks/barbs work too, but have slightly lower chance). You cannot craft rare polearms, unfortunately, so save them when you find them!
• Run speed rifts in a group
  • If you really want to maximize your exp gains, consider playing in a group. Because of monster health difficulty scaling, as long as the people around you can solo-complete the same level gr as you (or are supporting at the same skill level you're playing at, a little bit harder to quantify though), you will gain more exp/hour by playing with them regardless of the group composition you use.
  • Mixing classes will generally increase xp/hr and clear time due to the natural synergy of party buffs and damage debuffs on monsters.

Advanced Strategies

• Follow the FOTM (flavor of the month) 4-man group composition
  • If you really want to maximize your xp gains, follow a build within the strongest 4-man composition and use speed-rift gear with a good 4-man group.
  • Hit Shift+L in game to see the leaderboards, look at 4-player and "View Hero Details" to see what items/skills they used to achieve their highest gr completed. This will differ from their speed-rifting builds so keep in mind that pushing gear is different than speed gear. Experiment with your own speed gear to maximize xp/hr.
  • Right now, there's a very popular Energy Twister DMO wizard composition out that can speed-rift gr100s and achieve up to 1 trillion xp/hr.
• Try not to waste time in between rifts
  • This very likely kills most players' xp/hr more than anything else. If you run a 5 minute rift and screw around for 5 minutes you've basically cut your xp/hour in half.
  • if you tend to do this: run higher grifts! This will force you to spend more time in the rift and thus waste less time as long as you're taking the same length breaks.
• As aforementioned, experiment with or look up builds online for speed-rifting gear for your class/build.

Gameplay Tips

• Skip the rift guardian (quit the game, make a new one)
  • Killing the rift guardian generally takes anywhere from 1-3 minutes at higher grift levels. This can really kill your xp/hr, particularly if the rift itself was only 5-6 minutes long.
  • Skipping the rift guardian requires a lot of scrutinization because creating a new game can sometimes take longer than simply killing the rift guardian would, meaning you lose out on grift completion benefits as well.
  • If you're skipping the rift guardian, you are generally going to be running grift levels closer to your max gr completion (2-5 levels instead of 3-10). This is due to the nature of most builds being centered around AOE as opposed to Single Target DPS. Some builds naturally have both, but at some point Bane of the Stricken is absolutely necessary to kill the rift guardian in time.
  • Bane of the Stricken is primarily used for the rift guardian. Skipping the rift guardian thus frees up an entire legendary gem socket for the DPS class(es) in your group and further decreases your clear speed/increases your xp/hr.
• Know when to skip specific elite affixes
  • For example, arcane enchanted elites are notoriously difficult to kill, especially in tight spaces/caves.
  • Know when you should skip elites, rooms with not enough density, etc.
  • These things come with experience/skill. In general, skip elite affixes you find yourself having lots of trouble with. Pay attention to the skills/mobs that kill you and avoid them.
  • In group settings, have 1 player designated to call out when to skip rooms/mobs/elites by saying "skip" and everyone in the group will move past that room and continue the rift. Generally, this is the most experienced player, but it's better if it's the player with the most skills designated around gathering mob types. This person is usually a monk with Cyclone Strike (a class used in almost every group composition), as they are the primary mob aggregator and thus determine where your group will be fighting at.
• Understand how to aggregate mobs to minimize clear times
  • This is beyond the scope of my answer, so refer to other guides online. Here is a decent one on how to build up density.
  • One of the most basic tips of aggregating mobs: if you find a pylon that particularly benefits your class, gather mobs before using it.