
I've seen this posted quite a bit - haven't tried mods myself yet. Saw one comment on a Youtube video said it was to do with the fact that the Steam launch command was essentially asking it to log to a console, slowing it down?
Far from a developer/modder myself so not too sure lol
edit: Like Toreador said, do you have the game installed on an SSD or HDD? I found a post on the Discord where they think they worked out the problem was having it on a HDD. Suppose this makes sense with the logging to a console thing too, as that would obviously be way slower on a HDD than an SSD.

it's installed in an HDD that never got an clean reinstall in over 2 years since I bought my pc
I do have an SSD but I didn't plugged it yet (still in original box as if I just bought it)
I plan to do a clean windows install soon and plug it in but just thinking about doing backups, having to reinstall everything back, etc is already enough for me to leave it for "tomorrow"
the mods works fine and the game has no lag at all, it's just at the start when icons and textures takes a bit longer to load and audio gets cut while loading a heist, besides that I have no issues

@berkrider
Interesting, it is likely that is the culprit! Let us know how it runs once you've moved the game over to the SSD.

I also have this issue. It works flawlessly (and with fast, almost non-existant) loading times, and i've got it installed on a 4TB X300 HDD. As soon as i enable mods, the game hangs when matchmaking and in the first 1-2min of the heist. After that, it behaves as normal.
The weird thing is that Ready Or Not is very similar in both game engine and it having mods, but i've got about 10-15 mods and there is not a single issue or delay with them on.
But it's early in PD3's release, so thought i'd just leave this comment here since it'll probably be improved.
I don't think it is the mods that are causing this. Most likely servers or in worst case where you installed the game (If you put it on an HDD or not)