All other things being equal, more megahertz is faster. Problem is, how much faster isn't exactly a linear function of clock speed. The system can burn through instructions on a 50MHz 030 42% faster than it can on a 35MHz 030, yes, but the instructions and their associated data have to get to the CPU first. Unfortunately, most Amiga accelerators were built at a time when main memory just couldn't keep up with even 35MHz, let alone 50MHz (at least not cost-effectively,) so the CPU can be kept waiting for memory to respond, and since the memory speed is relatively constant for that era, the number of cycles it wastes spinning its wheels actually increases as the clock speed goes up.
There are mitigating factors, thankfully - the 68k architecture has always tended towards longer cycle-counts for instructions anyway, so even a fast CPU isn't always going to be running up against the memory bottleneck for every instruction (especially not math-heavy stuff.) And the 030 does include a little bit of high-speed cache memory (holds frequently-accessed instructions & data, and serves as a buffer between a fast CPU and slow memory.) Still, the 030's cache is small, and very few Amiga accelerators include any additional cache, so it only helps somewhat. (This is part of why an 040 can make such a big difference, it has much more cache memory.)
