I've found that the more rules depth you use, the more your players need to buy into and learn those rules.
It's not the rules system, it's the depth and number of rules. I've played many games, most heavily GURPS, 1st edition AD&D, and Rolemaster. I can't say that any of those is really more rules heavy than the others. GURPS has a lot but they're mechanically more similar to each other and generally are modifiers and special cases. AD&D is a mess of different systems and special cases and a mix of those two. Rolemaster is just heavy on rules lookups, even if the system is remarkably simple (roll high, open-ended, add modifiers, consult a table). Anyway, that is a tangent.
But the more rules we enforced in play, the more players needed to know those rules.
Basically rules have weight, multiplied by the number of players, and that weight is divided amongst the players who know the rules. The fewer that know them, and the more rules, the more the game is slowed by rules adjudication issues.
You could say:
Rules Weight = Rules Used x Number of Players / Players who Know The Rules
A good example of this is my DF game. It's rules-medium.
It's not rules-heavy, as we've tossed many special cases and optional rules out of the window. We only use rules if they fit the specific challenges of the game. Damage to items, special grappling rules, special rules for combat, hundreds of magical spells, etc. are all in. Many other rules are out.
Even so, it requires a lot of player buy-in to learn the rules. We get this to a varying extent, from people relying on memories of previous play to those meticulously looking up the rules. The less people learn the rules, the slower the game plays. People need to deal with combat modifiers, defense and offense rules changes, spell lookups, effects of stats and skill levels on effects of spells and rolls, etc. Since we don't get 100% buy-in on learning everything, it slows things down.*
On the other hand, GURPS Gamma Terra aka 20th Homeland aka Gamma World+ (it's Gamma World, don't doubt that for a second) is rules-light.
Our character sheets, one equipment list, and one handout page covers all of our skills and abilities, stuff we can buy with XP, our campaign details, combat rules from range bands to ROF, etc. Maybe once a session we need to consult rules about setting things on fire or Aim bonuses or shooting from Prone. That's it. This has low player rules knowledge requires. Nothing is really slow, because there isn't anything to look up and very little to consider. Similarity between PCs means we can tell each other the required rolls.
My old GURPS game was more rules-heavy, and needed even more buy-in. You needed to invest more time in learning how the rules worked just to offload the responsibility of knowing them from the GM and other players. Someone had to know the rules if you didn't.
So whenever I think about rules weight and adding or subtracting rules, I do it with this idea that the players need to put some work in. If they're not going to do it, even a mechanically superior and campaign-superior rule is going to subtract more than it adds.
* We're hoping a switch to the DFRPG rules set with a single house rules handout for add-ons will help this. I expect so, but it won't be perfect. Players will still need to read the rules set to make the play speed as fast as it could be!