The overfilling problem is the very small seal at the end of the lever which is lifted by the float not closing properly. I have the same problem.
Macdee will supply a complete fill valve assembly, but all that is needed is that tiny rubber seal. What a waste of material!
I made one of my own from a larger piece of rubber, a tap washer I think. It worked fine for a year after settling in, but it has started leaking again. This time I found a soft brown gunge below the seal, which I cleared, but it still leaks.
The slow filling is probably due to dirt in the flow restrictor part of the assembly. I also found that gunge on all these parts. Unscrew the large knurled plastic nut, remove the whole (you have turned off the water first?) and disassemble carefully, noting the way the various seals and plastics bits fit. Clean everything, reassemble and refit the nut - use some PTFE tape on the thread to be sure it does not leak.
Flow restoring is easy, but that seal!!!
Peter Miller, December 2010