[Opinion] The reason why ADC's are weak early game is because of the runes they typically take

Why Precision sucks

Precision is quite easily the second weakest rune page, ahead of inspiration; it offers no attack damage or significant increases in damage, particularly in the early game (12% attack speed is literally just a dagger). Lifesteal is pretty much irrelevant, as ADC's currently have plenty of options for sustain And the tenacity rune is frankly a joke. What about the end of it? well, the only viable rune at the end is the +7% damage when targets are below 40% health... Again, not very impactful. If you think about it, let's say you're dealing 100 damage per second (which is extremely generous for ADC's going precision), now you're only doing 107 damage... this item is often less impactful than a goddamned in the early to mid early game, the most important time for an ADC due to its vulnerability. Overheal? Lethal Tempo? Fleet Footwork? All of these runes are now only good in the late game, again the time when all that frankly doesn't matter as ADC's scale like gods. The exceptions are clearly conqueror and Triumph; these runes are decent in the early game. However, Triumph in actuality isn't very good on Marksman, as it is actually extremely difficult for a marksman with precision to kill anything in the early game. So, in other words, the only good early game rune in precision is conqueror... A rune that isn't designed for ranged characters and likewise is basically only good on Draven.

tl;dr: Precision runes suck major dick in the early to mid game for marksman, and likewise marksman would be better off going for different runes.

Other options For me, the most obvious best early game runes for Marksman actually lies in domination You go Hail of Blade, Taste of Blood, Ghost Poro, Ravenous Hunter Why are these runes good? Well they are more geared to the early game; none but one of them scale up in power. What you get is what you need: hail of blades wins you most trades and is essentially an instantaneous version of lethal tempo; Taste of Blood and Ravenous hunter ensures you have perma sustain, much stronger sustain that Fleet Footwork; and Ghost Poro ensures that your lane will Always be warded. Moreover, it's also great for low elo players, as they have a harder time looking at the map; ghost poro will ping the map if an enemy walks over it, making it to where low elo players can actually utilize vision.

The next rune for selection is much more difficult, as sorcery and Resolve are similar in power level. I'd say go Sorcery if your lane opponent is a mage or another marksman, and only go Resolve if you're against an extremely aggressive laner such as Wukong or Quinn

Other Perks of going domination The attack damage, in my opinion, is 100% more useful than the attack speed, especially in the early game. It magnifies your lifesteal and honestly it's better against all these burst champs that want to combo you then walk out. And also, in my experience, the extra attack damage is more helpful with CSing than attack speed.

Tl;dr 2 Dominion is the superior page because it does everything precision does, just better, particularly in the early game; more damage, more survivability, more vision, etcetera. The sacrifice in attack speed is more than worth it.