I think a steam user might have found the solution to the severe lag you get from having vehicles with their emergency lights on (especially bad with modded american vehicles with lots of lights) and some AMD cards, at least mine and a few others.
i recently got an AMD 5700XT, swapping over from a decent Nvidia card, and my framerates have gone through the floor. Just now i found a post on Steam Discussions on the EM4 Deluxe version explaining that changing a string of code in the Em4 cfg file solved their problems.
Simply change
<var name="e4_mbfastcopy" value="1" />
to this
<var name="e4_mbfastcopy" value="0" />
ALL CREDIT goes to BetterWarrior and his friend for solving this issue, the original post is linked below, if you want to go thank him if it works for you.
https://steamcommunity.com/app/757210/discussions/0/2935742047961399361/
I just tested with the Harbor City mod, 12 fire engines all with their lights on, and my game held around 25-30 frames. before applying the fix i would get around 10 with just one engine on screen.
If anyone knows what the snippet of code does actually do in-game, i'd curious to know.