Right now you are calculating all trains every 30 ticks, it makes small lags by megafactories.

Can you please change it as not more than one calculation per tick? Such solution makes no UPS lags by megafactories, but the distance calculating will be a little bit not so precise. The same precision will be by #trains = 30.

You can also get some more precision if you use the average speed:
distance [meters] = (0.5 * last_update_speed + 0.5 * actual_speed) * ticks_between_updates

This is possible, but have you tried unchecking "Update train distance every tick?" in the mod settings? This will switch the UI to refresh on demand which will save some performance. The calculations are still done in the background, but not updating the UI every 30 ticks makes a big performance saving, so give that a try.