Just to add to this; I don't think you need to go and buy anything else. If you have a mower than can cut 1.5acres in 15min...you have the correct mower for the job.
You merely need to increase the cut frequency as other have said. Your Iseki is ideal, but to keep it short and neat, you do need to cut it at least once a week (Based on my findings in my area). If it looks like it needs cut, you're too late.
Cut the grass slightly longer than you would normally. Cut it to about 2.5" or 3", that will give the clippings somewhere to fall into to hide them. Ideally only cut 1/3rd of the leaf, so that rule should dictate your mowing frequency. Vary your cut pattern.
If you don't have a mulching deck, try splitting the area into sections and cut it constantly throwing the clippings inward (Onto the uncut grass). Then do the last few stripes outward to kick it back out. This means you are constantly chopping the clippings finer and finer. You may end up with a few heavy strips at the end, if so just lift the deck a click and run over them to disperse evenly. Don't do massive sections at the start til you get into the way of it and get a feel for how much you can throw in.
I found this works well to leave a clean cut and clippings evenly dispersed. In wet weather - its going to tend to clump, so possibly cut more often and discharge onto uncut and live with it!
Grass is 70% moisture, so in a day or so it does wilt considerably. But mowing twice a week (Possible if its only 15min) should be perfect to maintain it immaculately with no visible clippings.
Going from 2ft tall to 2", not collecting, is just too much grass to hide. You really do need to get rid of it that first time. As this was a few days ago, it should be rightly wilted down by now, so you may be fit to run over it again to break up any clumps and it should break down. But ideally yeah, you'd want to get a machine, or someone, in to remove that inital volume off for you. If you're on your own and its still lying everywhere, use the Iseki and cut it in such a way to throw the clippings into a few rows, then rake into builders bags or a trailer. You may need to rake out clumps. Bit of elbow grease the first time I'm afraid.
After that though, your Iseki is perfect to keep on top of it.
