Fastmem 4 Voices Configuration:

  Here you can set the size of the "Chip Memory Buffer" which is the amount
  of  CHIP memory used for every channel while playing from fastmemory. The
  default is 512, which suits  most configurations,  but if  it  gives  you
  trouble, you can try to change it.

  But remember that a smaller  buffer  makes  the  processor  work  harder,
  because  it has to copy the sampledata much more often than with a bigger
  buffer. But still, a big buffer can give some troubles, so try it out for
  yourself.