My new (second hand) slalom board has the finbox further forward than my previous one. This provides a secure ride, but is a lot slower. How can I make this board faster? Same fin but bigger? Fin with more rake? Softer fin?

This is a verrrry hard question. Depends upon tail width, bottom shape, all sorts. What are the boards, specs, and tail width

Besides the questions asked, we also need to know if you are able to trim the board a correct angle, ride the tail on demand, sail in a straight linen and how uncomfortable you are at speed.
Set up questions arise, as most boards with forward fin box will have forward mast tracks.
So measurements.
Set the 2 side by side and look for obvious differences.

And usually, a more secure ride means the finbox is more towards the tail, if that is the only change.
But you said forward.

Buy 20 year old freeride gear , it's faster

You still don't understand, do you?
The actual year of the gear matters little.
It's matching the right gear with the right skill level that matters.
Plenty of us oldtimers had the WRONG gear to compete for fast times.
Some had the right gear.
Our differences in speed between the best and worse was much greater than NOW....because YOU have become better and smarter as you aged and gained experience.
I already mentioned I might have peaked, as a windsurfer, possibly 30 years ago. Meaning, downhill.
How is it possible, using 10-25 year old gear, I can stay with younger, bigger, stronger sailors on the newest gear.
I already said I get walked on coming out of jibes.
I also lose speed in the biggest holes.
But during gusts I'm right there, and in higher chop usually edging away.
On old fast freeride gear and old sails, mostly 1-2 meters smaller.

I'm just yanking your chain LeeD.
Payback for not understanding half of what you say

Thanks for all the replies. Unfortunately I don't own the old board anymore. I know the tail is wider and has more volume. But let's assume two identical boards and one has the fin box further forward. Would one need a bigger/softer/more raked fin to get the board more out of the water?
ps don't need any more info about LeeD's amazing skill level

Why don't you just tell us what board?
Someone here probably has ridden it and have it dialed in?
Most cases, moving box forward gives more response, should lift the board more out of the water, go faster, be a little harder to control...if the fin holds in.