It’s never going to be all that quick on a full sheet but and more intricate design and more cm of cutting of course will play a big part too. If you’re pocketing at all a bigger bit will reduce the time for larger pockets if you get to tool changing or setting up different programs.
As the milling basics suggests reducing the number of passes is another ticket to reduced time. In your case for an 18mm sheet at your 3mm doc you need to make 6 passes (assuming you are going exactly 18mm for a full cut) and then for finishing you need two more passes at 9mm doc for 8 passes.
You mentioned coming to a sweet spot for your feedrates but there may be a combination of doc and feedrate that can get you cutting deeper ? A 5mm doc would drop 2 passes on your initial cut. If you were to use a longer bit for the finishing pass you could probably do the finishing pass in one single full depth pass if you had the cutting edge for it. These two would get you to 5 passes total from 8. If you were to cut just a little more, 6mm would get you 4 passes total, halving the pass count. Each machine is different but many of us cut 5-6mm doc without too much trouble and the full depth finishing pass is wonderful and an easy time savings.
I saw your post about the collet slipping. I ran into this myself and my solution ended up being a new collet. I couldn’t see what the problem was with the elaire collet but after abusing it one too many times in my error & trial phase the bits would just pop out in seconds to minutes no matter how tight I did it. Since switching collets I’ve never had an issue.