Warning, a long theory and GM/referee advice post follows. Another warning, some of this advice is repetitive, but so be it.
This, as again you can see from the numerous posts about it, has been wonderful and rewarding. But I definitely have had to make some adjustments to the WFRP rules-as-written to make the game play fast and fun. I thought I could summarize some of how I have been running the game into some general principles applicable to most games on the heavier-crunch/complication side of the spectrum so that they play more like a fast and imaginative simple, uncomplicated game like B/X D&D.
The biggest problem crunchy games like WFRP suffer from are:
1. Rules Bloat: Remembering how to-hit, damage, wounds, healing, combat manuevers, feats/talents/powers, and spells work and work together can slow everything down when you want to have the most excitement: life-or-death combat. I suggest addressing this with referee familiarity, quick sheets, and making a decision and moving on.
2. Decision Paralysis: The number of rules for stuff your character can do often leads players to look for the sheet for answers instead of coming up with in-game solutions, and not knowing what to do when its your turn in combat. I address this in my game by calls for action, asking them to describe actions (not use game turns), and moving on.
3. Constrained Thinking: Similar to the above, thinking in terms of game mechanics (since there are so many) instead of in-game and real-world thinking. Rules-heavy games can be great toolboxes, don't treat them like a computer system or computer game where everything should be addressed from the rules' perspective. Instead, imagine whatever you want and let the rules of the game help you determine a fair way to resolve things.
Overall though the advice of the late Dave Brockie of GWAR is the best and will see you through: KEEP IT MOVING.
So, here are DM Nick Longshanks's principles for running a rules-heavy game like WFRP (or D&D5e, etc.). These principles are for the sake of fun while playing a game. Don't let the game rule you, you rule the game.
1. Keep it moving! Cannot be said enough. Do not stop the game to look something up unless its really crucial/worthwhile like a PC's critical hit or otherwise evening-making/breaking. Call for actions. Have enemies attack or arrest the PCs if they're dithering. Make 'em roll Endurance or get the plague if they're farting around. Decide what a rule will be if you cannot remember and no body else does; you'll correct it later or whatever.
2. Only call for rolls if something interesting or unexpected will happen if the character fails. No Perception test needed for listening at a door (they're already listening at the door, jsut tell them what they f'ing hear!). No Charm test to see if the barmaid likes you (who cares). You don't roll Ride/Dex/Agility for traveling, you do it when wolves attack your horse or you're racing away from a knight or whatever. Every roll slows down the game and breaks up the flow of player interaction, jokes, and imagination time, so make them interesting and worthwhile (ie. they should affect the plot outcome). Most published adventures and games for these systems are filled to the brim with bad examples of boring as paint-in-the-numbers skill checks that a just a snooze fest. Look, any RPG sucks donkey if its just you sitting around rolling dice that don't mean anything while the GM narrates stuff. Get away from that by: (a) telling the players what happens, no need to slow down the narrative or dialogue with a roll (I look under the carpet, I knock on the door, I greet the NPC-most gameplay does not require rolls).
3. Players describe what they’re doing, you tell them what to roll or roll for them if it’s secret or simply tell them what happens. This is related to the above. Do not stop players asking for different stat to roll if it makes sense. Think about what the roll really means: it's simply a randomizer based on the in-game character's fiction to generate interesting results. Don't make the characters memorize their character sheet, their concept should tell them and you as DM all you need to know (really). So call for descriptions of what they do, you tell them what they roll or if nothing interesting happens if they fail the roll, just tell them what happens.
4. If you don't know the correct rule, make a ruling! Rather than slow down to look something up or argue, make a ruling and move on. If it was wrong, explain later and announce the rule going forward. If this results in house rules, all the better. You're much more likely to enjoy a house rule than a "condition" rule buried in the rulebook anyway.
5. Know how to run a combat, cast a spell, and heal well. These generally are what go beyond a base mechanic like roll under percentage or roll high on d20. Understand the basics of combat (surprise initiative attack defense damage healing death) and spellcasting/magic. Don’t worry about fiddly exceptions until you’ve got some basic combats under your belt. Run some mock combats by yourself before gaming, and have basic foes before introducing more advanced foes with special abilities and spells when you're playing with characters.
6. Have reference guides. Make some quick sheets for yourself and the players with character creation outline, combat, spell casting, equipment list (damage, armor, costs) and common monsters/NPCs. This last part is really your basic setting/campaign creation, and you can make random encounter charts off these. Then when writing adventure outlines you can just write Cutpurse or whatever, and reference it. Making these yourself is helpful because then you learn the complicated rules. But if you buy these, it's fine, just anticipate fudging and making a ruling and moving on more at the beginning.
7. Decisions have consequences. Don’t pull punches to preserve the ‘story;’ the story is what happens due to players decisions, and if you give plot immunity, it’s boring for everyone. This also preserves the unique thing of the medium of RPGs over movies, books, and video games: your imaginary characters can do anything but you have some rules to introduce the tactical and decision-making/problem solving bits that makes any game or sport fun.
8. A description is more important than a mechanical effect. So if a character has the Carouser or Attractive talent, that is more important as a description than the mechanical effect of a bonus to certain rolls or whatever. The feat should not be driving a call for rolls, it should simply have the natural consequences. So an ‘attractive’ woman should have no problem drawing attention and distracting a drunken man, no need to roll (unless there is something interesting like a failure means learning he’s not attracted to women).
9. Disregard "canon." To the extent you have a fictional world with a lot of backstory and 'canon,' ignore this at whim. You're doing all the work of running the game for your (ungrateful) friends, they don't get to one-up with some shit some stranger asshole made up about elves. Part of the fun for all DMs is making setting background stuff and plots, so don't get that derailed by some know-it-all player or worse yet, stranger working for a company, to tell you how to imagination-elf-game. That's some middle school shit no body should have time for. Sure, if you all agree you're sticking by Tolkien or Warhammer 'canon' and that's part of the deal or fun for your friends and you, sure, but the default should be that what the DM says rules.
Folks, you're playing a roleplaying game, not a movie or videogame. Use the fact that the DM is a thinking, human being who you're playing with. You don't need to outsource your friends' imaginations to a rulebook, which is a tool, not a cage. If you're using a game with a lot of rules, the latter is always a danger moreso than rules-light games where it's intrinsic that you won't have rules or rolls for lots of situations. But if you run the rules-complicated game more like that, you can have just as much fun but without needing to make up a spot/house rule for as much stuff. Also, I enjoy different weapon/armor interactions so there. Happy dicing!