One of the interesting things about the long-slide Glock 17L competition pistol, is that Glock was able to engineer it to reliably function with the same recoil spring assembly used on the shorter Glock 17, despite the longer slide and barrel. This was done by removing as much excess metal as possible from the 17L slide assembly, by machining cut-outs and recesses in non-critical areas of the slide. By getting the weight down to nearly the same weight of a model 17 slide/barrel assembly, the same recoil spring could be used without compromising reliability.
The photos below are of a late Gen1 model 17L and early Gen2 model 17, manufactured a little over a year apart (side-profile photos of these two guns can be seen in posts number 7 and 18, above). Although the frames were quite a bit different, as far as I can determine, there were no changes made to the basic Glock 17 slide assembly between these two models, so any differences are attributable to the machining required to lighten the slide for the model 17L.
The top view shows the most obvious difference; the "window" in the top of the slide between the front sight and the ejection port. This was not only for weight reduction; some 17L models were shipped with an optional ported barrel, which directed hot expanding gasses up through ports cut into the barrel, and out through this window, helping to reduce/suppress the tendency for the muzzle to rise or flip during recoil. On later Gen3 17L pistols, this window has been moved or reduced slightly in length to allow a larger base for mounting longer front sights, such as the light-pipe sights often used in action-shooting competitions.
Flipping the slides over, we see the first major internal weight-reduction cut, along the lower-right edge of the 17L slide. This cut is deep, extending very close to the top surface of the slide.
On both sides of the slide where the breech (or chamber) portion of the barrel slides up and down during cycling, the slide is much thicker, to guide the barrel into position as the slide opens and closes. These thick areas were reduced by machining the solid slab areas into "ribs", removing metal in two different spots so the barrel still would be correctly guided into position, but without using all the metal left in place on the model 17 pistols.
Right-side view (17L slide at the bottom):
and left side view (17L slide at the top):
These lightening cuts allowed the early Glock 17L competition pistols to exhibit the same high level of reliability as the service-grade model 17s, with the added perks of an extended sight radius and a lighter trigger connector, making them one of the first factory-issued long-slide high-capacity competition pistols on the market.