A large fleet or a lot of missiles will be very slow in Reassembly, but it does not make full use of the CPU.
Now a lot of gaming PC has at least eight CPU cores, if optimized for multi-core, the game's performance will be greatly improved

Reassembly would probably never lag again.

It would be lovely, but I wouldn't anticipate it. I'm pretty sure it would necessitate a full engine rewrite, which is a pretty big ask.

You can try some multi thread software , even nvidia control panel can help some