Pull the tack line fully in. Looks like there are 3-4cm of luff tension to gain there

More luff tension allows you to sheet the spi out more to go faster and deeper (to a certain point at least).

If you do the "grab the luff in your fist and twist it" test. How far can you rotate your fist? 45deg? 90deg? More?

Hwo did you set the luff tension line inside the spi luff? On a new spi I hoist the spi with a slack tension line, then tension it about 5cm.

When sailing with a spi, it is very easy to sheet in too far and close the leech. The leech needs to be open and complement the mainsail trim for optimal speed. The rule of thumb is to sheet out as far as you can, until the luff curls, at all times. Only experience will tell you when you are on the optimal course. If another boat walked over you, try to replicate their setup

An old spi will become deeper as it ages and is used. A new spi is flatter and faster. If those other guys had a new spi and your spi is well used. Well..