automount problems

I have a Yosemite Mac Mini with performance problems. When looking at the disk activity with opensnoop I noticed that automount is performing tons of accesses as follows

  UID    PID COMM          FD PATH
    0   1124 automountd     4 /etc/auto_home
    0   1124 automountd     6 /etc/auto_home
    0  55536 od_user_homes   5 /System/Library/OpenDirectory/request-schema.plist
    0  55536 od_user_homes   7 /usr/libexec
    0  55536 od_user_homes   7 /usr/libexec
    0  55536 od_user_homes   7 /usr/libexec/od_user_homes
    0   1124 automountd     4 /etc/auto_home
    0   1124 automountd     6 /etc/auto_home
    0   1124 automountd     6 /usr/libexec/od_user_homes
  501    464 syncdefaultsd   4 /Users/corti/Library/SyncedPreferences/.dat01d0.559
    0   1124 automountd     6 /usr/libexec/od_user_homes
    0     58 opendirectoryd   9 /var/db/dslocal/nodes//Default/users/corti.plist
    0  55537 od_user_homes   5 /System/Library/OpenDirectory/request-schema.plist
    0  55537 od_user_homes   7 /usr/libexec
    0  55537 od_user_homes   7 /usr/libexec
    0  55537 od_user_homes   7 /usr/libexec/od_user_homes
    0     58 opendirectoryd   9 /var/db/dslocal/nodes//Default/users/corti.plist
    0  55538 od_user_homes   5 /System/Library/OpenDirectory/request-schema.plist
    0  55538 od_user_homes   7 /usr/libexec
    0  55538 od_user_homes   7 /usr/libexec/od_user_homes
    0  55539 od_user_homes   5 /System/Library/OpenDirectory/request-schema.plist
    0  55539 od_user_homes   7 /usr/libexec
    0  55539 od_user_homes   7 /usr/libexec
    0   1124 automountd     4 /etc/auto_home
    0   1124 automountd     6 /etc/auto_home
    0   1124 automountd     6 /usr/libexec/od_user_homes
    0  55540 od_user_homes   5 /System/Library/OpenDirectory/request-schema.plist
    0  55540 od_user_homes   7 /usr/libexec
    0  55540 od_user_homes   7 /usr/libexec
    0   1124 automountd     4 /etc/auto_home
    0   1124 automountd     6 /etc/auto_home
    0   1124 automountd     6 /usr/libexec/od_user_homes
    0  55541 od_user_homes   5 /System/Library/OpenDirectory/request-schema.plist
    0  55541 od_user_homes   7 /usr/libexec
    0  55541 od_user_homes   7 /usr/libexec
    0  55541 od_user_homes   7 /usr/libexec/od_user_homes

Hundreds of access per second. Is this normal? What could be wrong?