You need to find out how long time it takes to load a records from SD-card into memory. When you know that, you create a buffer/array of records and pre-load as many records as possible (or needed) into that array before starting the sequence. Whenever the delta time is large enough to load one or more records, you do so. You will need to handle the array as a circular buffer where the beginning and end is defined by variables in order to know where to put new and get old data.