While importing the added list of PMID entries I noticed the following.
1. first trial jabref choked, with a memory usage of 3.9 to 4 GiB observed in the Task manager, never finished and the import dialogue didnot disappear. The Jabref program had to be terminated in the task manager.
2. second trial worked, however the memory usage was raised to 3.4 GiB as before, when the import was completed, no memory was cleared.
3. Exiting jabref and restarting it showed the normal memory of 460.000K.
The extra memory used while importing should be released as soon as the import process is finished. That it is not, points to programming errors.