KSP: 1.3.1 Windows 7 64bit
Problem: Same problem as the gentlemen above me. Extremely low FPS whenever solar panels are present. Testing using some stock ships with high part counts is fine, but as soon as solar panels are added it all slows down. All of my testing was done on using stock equipment.
Mods installed: [all current] Kerbal Engineer , KSP Interstellar Extended, KIS, KAS,
Reproduction steps:
1) Make vessel without solar panels and test it. Everything should work fine.
2) Add 2+ solar panels on vessel. Attempt a test launch. The game will slow down dramatically.
3) There is no change in fps in the VAB, however. (I just copied and pasted the guy above me since it's the exact same issue.)
Since all three of us are using Interstellar, I'll try the game again after removing it to see if it still occurs.