Ref limit in browse

I try to implement a Random mode for mopidy-subidy extension see here
but when I use a random vdirs if I don’t put a limit of return ref item to 80 I have a timeout on ncmpcpp and a log
DEBUG 2020-11-24 15:43:56,229 [1091:MpdSession-32]
Client most likely disconnected.

to test I have create a random mode with search and no problem to find 500 item with this method. do somebody have an explanation on that?