Summary data is usually available as SMSPEC and UNSRMY files. UNSMRY files are stored in a binary format, and all data for one time step is stored one section. When extracting data for all time steps for a single summary vector, data must be read from multiple sections. This can give bad performance for large datasets.
*.ESMRY files contains the same data as SMSPEC/UNSMRY. The data in these files store all data for a summary vector in one section to give significantly better performance when accessing single summary vectors compared to SMSPEC/UNSMRY.
ResInsight will by default use this file format.
If no ESMRY files are available, ResInsight can produce these files. This can be configured in Preferences. Note that all summmary data is read and write into a new ESMRY file. This operation will take some time, and can take minutes for a large ensemble.