XQuery performance degradation as number of json files grows

declare namespace _="http://com.sap.ariba.metadata.event/log";
declare namespace log="http://com.sap.ariba.metadata.event/log";
declare variable $logFilename external := "2017-01-31.xml";
declare variable $logDirectory external := "log";
declare variable $dbSchema external := "schemaRepo";
declare function _:write-log-event() as empty-sequence() {
  let $event :=    <entry id="6669" timestamp="2017-01-31T14:24:49.83-05:00">      <command toPath="meta/services/demo/serviceversions/v1/docs/demo/biggerform.json/1" isBinary="true" dbSchema="schemaRepo" type="ADD_BINARY" ></command></entry>
  let $logPath := $logDirectory || '/' || $logFilename
  let $fullPath := $dbSchema || '/' || $logPath
  return insert node $event as last into
           doc($fullPath)//log:log
};
declare variable $tempFileUri external := "temp/7b04aab6-6ee6-47c7-89e0-e5132e505daf/meta/services/demo/serviceversions/v1/docs/demo/biggerform.json/1";
declare variable $toPath external := "meta/services/demo/serviceversions/v1/docs/demo/biggerform.json/1";
if (not(db:exists($dbSchema, $logDirectory || '/' || $logFilename))) then (
    error(QName('http://com.sap.ariba.metadata/err', 'LogFileReq'),
	"write operation logFile not exist.")
)
else (
    try{
        db:rename(
           $dbSchema,
           $tempFileUri,
           $toPath),
        _:write-log-event(),
        db:output("SUCCESSFULLY")

    }
    catch * {
       fn:string("FAILED" || ": " || $err:description)
    }
)