Scalextric is a brand of slot cars, those little toy cars that race around a groove in a track. They were released in 1960 (You should check out some of the vintage Scalextric sets if you haven't seen them!). So these improvements to Scalextric sets will probably work with most slot cars but just so you are aware they are focused and design for Scalextric.
Now I'm a big believer in you should measure before you improve and in this case it a fun too! So grab some track and build a simple course the cars can't fly off at high speed. Grab to cars and get them to race around at the highest speed. If they don't finish at the same time have a go though a few of your cars until you find two that do. That's our baseline! So do the modifications below to only one car and one side of the track and see the difference.
Now before you go modifying anything make sure you aren't messing with any Vintage Scalextric cars. These can bit worth quite a bit of money in their original form so don't mess with them. Hell, send them to me and I'll send you two new ones ;)
Let's start with the car because that's where you can get the biggest benefits. Let's start with the best mod, a new motor. The motor that comes with you normal cars runs at X revolutions per minute (rpm's from now on) but you can get motors of the same size that run at up to Y rpm. So you can take out your old motor and put a new one in. Of Course the fast your motor goes the fast your car!
So let's get Modifying! When thinking of how to improve performance in a Scalextric car it's the same as what you would do to improve your own car, better motor, tires, roads etc. In fact if you know anything more about cars than I do (and that's probably likely) then you probably have a few more suggestions that I do so please send them on but here's some that you can try.