Sloooooow turn signals

It's usually from low voltage caused by old wiring or a failing battery or alternator.

What's your battery voltage at idle and at 1500+ rpm? What's your battery voltage when the car is turned off?

It's possible for the flash rate to change with too much load (like trailer lights) but it usually changes the behavior too. Instead of the bulb being on/off 50/50 it can flash fast with a long pause or flash long with a short pause. By flash I mean the bulb turning on, and by pause I mean it turning off.

At any rate, I'd start with your voltage levels when off and when running.