You have to tune your system for FSLTL to run smoothly in worst case scenarios. There’s a good video about this on YouTube.
So try setting a FPS overhead budget of say 15fps for AI traffic and then dial back some of your sim graphics setting until you can accommodate that 15fps loss. This method works for me.
You also need to think about TLOD as it can hugely affect CPU perf especially around Ini’s KJFK. Which is quite well optimised for the level of detail it has. Note: you can also turn down KJFK graphical settings a bit via Ini Manager installer.
Also, turn down all other in sim AI traffic settings as they load your CPU a lot too and hog resources. So (ships, road, aiport workers etc all set to say 10 or less)
Finally make sure that you don’t have too much ai traffic enabled in FSLTL or have too wide a radius set especially on the ground in FSLTL as there are quite a few airports around KJFK and if your radius is too big, they will have AI spawned there too which will be a needless use of your resourses.
In the end it l boils down to resource management. FSLTL runs perfectly well if accommodated for.