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Is there any indication on the problem in arjavaplugin.log or arerror.log?
Do you have some attachment size setting on ar server or it is set to unlimited?
Can you try to create a testform with an attachment field (or use an exiting one) and attach one of the larger files to it. Do you experience the same issue?
Try enabling API/Filter/SQL log to one file on server, when you import such a file and check wheter there are any long running SQL operations.