I've run 5 sessions of Spycraft - 3 missions total.
During play there hasn't been any delay and it runs just as fast as BESM a little faster than D&D and faster than Feng Shui ran for our group.
I agree with Jims 20/80 statement. You use 20% of the rules, 80% of the time. Actually more than 80% of the time for my group, but we're purposefully taking off bites. We didn't want to start our first 3 missions with Stress damage, machine guns, rocket launchers, lasers, acid, standoffs, car chases, interrogations, etc.
If you try to use every rule at once right off the bat your brain will fry. While the 20/80 rule is in effect, the GM has total control over what goes into an adventure. So they could, if they wanted, introduce 100% of the rules in a single game if they had intention to. I think that would bog things down.
Choosing gear I think takes the most amount of time to learn for the least amount of time the rules are used. A player might need to learn 3 different subsystems to figure out how Gadgets, Vehicles & Weapons work. They're pretty simple, but it requires actually reading through the 5 page sections, not skimming over things. The other categories are extremely straight forward. You do need to know what you're going to pick and there are a lot of options here.
The initiative system isn't worse than normal d20, it's just a bit more tactical. Some Standard Maneuvers, Feats, Stances & Tricks modify your - or an opponents - iniative count. For lower level characters (I have 3 & 4) we have only a few init modifiers: Regroup, kicking & shooting guns.
The players might start to make use of more ticks and other stuff.
Honestly combats are very fast and some combats are only 20 minutes long because we stay in "combat mode" while in certain areas. I havn't had a terribly long combat yet and I've had to deal with up to 20 NPCs in a combat so far.
The longest slowdown we have is making plans becuase we have 6 players who range from Soldier to Faceman to Sleuth to Scout, so there are so many options on how to complete a single target.
There are times something comes up that you don't know a rule and rather than look it up you rule on it "I'm going to say I think this is how it should happen" and the players agree or make a quick case on it and you move on.
I think this book is deceiptively complicated looking because it allows for so much. You could boil it down to 200-250 pages and loose much of the more "advanced", "optional", and "special case" rules. Some people don't care for these things. Others do.
BESM d20 has the d20 rules boiled down to like 10 pages (classes, skills & feats aside! just the rules to play the game!). It doesn't cover or allow for anything specific to happen when you're hit by a laser vs a bullet vs a sword vs acid. It's just assumed that the GM will rule it if they care to.
So I wouldn't say that SC2.0 is complicated or bulky in play - but navigating the book takes some getting used to
