I've done most quests in the game, am on 3rd out of 4 DLCs, and my quest log is slowwwwwwwwwww to access. Any mod/cheat/fix for that ? Thanks.

Just your quests or anything in the Pipboy?

Try dumping excess items in your inventory into a container. Too many of those can slow down the inventory and barter screens, but I don't know if there is an effect on the quest list.

Does an earlier save have this issue?

Just the quests, and the Misc section to a lesser extent. My inventory is at a minimal (under 90 weight), and I don't have problems with those screens. Earlier saves are better because the quest/misc lists are shorter...

I have had a ton of quests going at once in my list and it never slowed down for me. I mean active ones, all the ones people complete are in the list.

So, the next question is, are these active or not started quests? Have you read all the Notes? (I don't think that would have any effect)

I even have HUD mods that give me even more information on the screen than the stock pipboy gives and it does not lag. If anything, the HUD is crisper and smoother in action.

They're mostly completed quests, I only have the endgame quest and the DLC quests I'm currently on (2-4) as active.