slow draining garbage disposal

Hi

I have looked aorund a fair bit for solutions for this problem but no solution seems to match my current issue.

As noted, my garbage disposal drains slowly. Interestingly, if I turn on the disposal when there is fair bit of water to drain out, it drains much faster.

I have checked the obvious things

Removed the traps and made sure they were not blocked. They were all quite clear.

Poured some water down the sink with the traps removed and noticed that the water coming out of the disposal directly flows quite well so it doesn't seem like it's blocked in the disposal itself.

The only thing I haven't been able to check is if the pipe after the trap going to the drain outside is blocked. This is because the house is on an elevated section and the plumbing goes down a fair bit of exposed pipe before connecting to the main sewer lines which go direclty into the sewer.

The only other thing I can think about is the pipe from the trap down to the main sewer lines in the house is blocked so I would need a snake or augur to clear it. But what puzzles me, why does the disposal drain okay when it's spinning. Is it some additional pressure down the pipe the spinning disposal creates?

Thanks for any hints before I have to call in a plumber