The FPU will not make any noticeable difference, except for niche software such as CAD packages or UNIX (or benchmarking utilities!). Other possibilities: more RAM needed? Hard drive full/ hard drive on the way out? Drive needing optimized?
As has been said, however, the very best way to speed the thing up is to run period software, and I would say that this should begin with the original System (7.1) rather than 7.5.5, 7.6.1 or 8.1. An LC575 running 7.1 can boot in maybe 25 seconds. If you max it out you slow it down. So keep it simple. If speed is the goal, small programs are not bad but good, as is elegant code generally. If, on the other hand, you are trying to run the likes of that bloated excretion called Word 6 on it, well, then you'll need to get used to going slow.
Though "fast" is a relative term, quite frankly an LC575 running the right software should not feel anything but fast. A 33MHz 68LC040 was not really a slow chip in the early 90s, but was marketed when the LC575 was manufactured in much the same way that a Core 2 Duo is today. I know; I was there.
But it was seen as fast in running OLD software, and everyone knew that at the time, because there was a problem implicit in buying the machine. By the time the LC575 was marketed, Apple had transitioned to PPC, at least at the high end of its lineup. Software was being written for the future, then as now, and companies (with the aid of new coding tools) consequently produced larger, more complex programs for PPC machines. An entirely new OS was promised, and it would be PPC-based. You had to write for it or lose your market. To remain viable in what was always a small market niche, however, they also needed to produce 68k versions of those same large programs, given the number of 68k machines still in use. Even Microsoft did it. The result was a dog's breakfast, especially for the small companies, many of which went belly-up.
Then Apple failed, and failed, and failed again to produce the promised Copland, things got worse and worse, and the collapse of the company beckoned. It was at the beginning of that era that the LC575 was manufactured.
The situation then is not unlike the difference between buying a Core 2 Duo today and a Quad i7 today. The Core 2 Duo will run a heck of a lot faster than an old 867MHz G4. But if we really do get software that takes advantage of multiple processors and multithreading, that Core 2 Duo is basically a dead end. Ten years from now, some kid will complain to an online forum that the Core 2 Duo he found in a junkshop seems slow, and that other ten year old computers seem much faster. And you, or someone else at a similar stage on life's way, will write back saying that if you run period software on it, it will run fast. And that, then as now, is the only answer it's really possible to give.