Performance mods vs the stopwatch

Setting your sag.  Biggest bang for your buck that is often overlooked.

$150 titanium clamp bolts from MXLocker.

I have a pretty built bike, but I'm not a good rider. Other than suspension and active maintenance I don't think anything has made me ride better.

If anything after I put some money into the engine it made me slower, it undoubtedly had more power but I was more wary of using it so instead of holding it wide I would short shift more. 

I bet for the avarage rider on a well maintained stock bike with suspension setup for them by a really good tuner and fresh tyres matched to track conditions you'd struggle to make any improvements to actual lap time that wasn't just a placebo

Just watch what Carson Brown can do on 20+ year old roached 2 strokes and you will quickly realise that all our 'mods' are a total waste of time 

But to answer seriously.... Tyres, sag, fresh brakes, properly aligned forks, cleaned and greased bearings, clean air filters, fresh oil, fresh top ends, valves in spec. Then with all that done, revalved suspenson. No point having a rocket ship of a motor if its in a chassis you aren't comfortable to push hard on.

fasthouse decals

Works Connection throttle tube.   Once you get used to it there's no going back.  It has the bearing at the end of the tube to keep friction down.   Once i got used to this years ago.....everyone's throttle feels gritty (which slows me down a bit).  Handlebars have to be the right bend as well.

Suspension...  setup, bars, sag, etc.  Tires.  It's almost never a mod to the motor...

Getting my suspension set up for my skill level and weight was by far the biggest improvement I've ever felt on the track. I can't ride a stock 450 engine to it's potential much less a modded one, so I never touch the engine (I don't even add a pipe or slip on). The next best thing for me was an oversized front brake rotor. The stronger the better (I got less arm pump). The one thing I want to try next is a Rekluse clutch, it's just hard to pull the trigger on $1500.00 for something I don't know will help. 

I’ll second suspension. I send my sons stuff to PR2 in PA and their motto is “it just works” and it really does. If you feel safer, you go faster is my experience. Not to mention not having to be quite so concerned about going a little short or long and it hurting.

Like others said good tires and everything being “dialed” and working properly.

No way that adds weight. Remove all decals even the factory ones. Scrapping off the spray paint helps to.

 But seriously, I agree with tires and suspension but correct control locations, I think, are often overlooked.

Suspension and a steering stabilizer, not always in that order. A lot of moto guys are missing out, stabilizers help in nearly all situations, especially if you’re not in the greatest of shape. 

Riding lessons.

100%

Basic Suspension set up, the kind you do yourself and quality coaching are the only things that make you faster

Like RussB alluded to, having your bike properly prepped is the biggest thing. All the horsepower in the world won't amount to much if your chain is dragging on the guide, the forks are leaky and the brakes don't work properly. 

Assuming all that is set, I've found that the biggest actual modification gains I ever got were from suspension revalving and a stronger front brake. I was an Intermediate before I felt I could harness those benefits, however. 

Maybe it’s just a mental thing for me but a gripper seat cover. If I ride a bike without one I feel like I’m all over the place and moving around too much. It’s probably not the case but it makes it harder for me to focus on technique. 

No way it's mental. If it doesnt have a good seat cover, at least 30% of my mental bandwidth goes to "hey, try to stop sliding back" and that interferes with "hey, go faster and dont die".

Anything that makes you more comfortable on the bike. Bars, tires, set up suspension (not necessarily revalve or A kit), sag, grips, levers and lever position.

Small stuff and many of those fall under the category of adjustments and not mod parts

fuel, lots of fuel

"Go faster and don't die!"

New rear tire.  

New lense for your goggles.

I can't complete a full heater lap to compare anything. So fitness

A KIT suspension and anodised parts. 

I remember JT$ saying if he to pick 1 single thing it would be spec tires. He said the amount of difference they make is ridiculous. 

1:  Riding lessons.  poor technique / form are why your lap times are what they are, and lap times wont come down with mod parts.

Lessons are relatively inexpensive vs unnecess mod parts.

Then Tires, and Suspension.

Ecu, sag, correct spring rate, suspension clickers, tires.

The correct answer here is to mod yourself.

I went from 270 to 170 pounds. No mods will make you as fast as you can be unless you're in the best shape you can possibly be in.

Kashima coating

Add to this the correct springs rates (fat boys only need apply).