Standard 25umID columns are packed with 3um C18 particles.
When operated with appropriate small void volumes, the average peak width at halve maximum (FWHM) in a 3% B/min gradient run will be equal or less to 0.035 min (2 s).
Note: Theoretically, the separation efficiency (peakwidth) in HPLC is independent on the diameter of the column. It is therefore quite remarkable that with 25umID columns slightly more narrow peaks are/can be obtained. It is believed that this is caused by wall-effects: the perifery/area ratio increases with declining tubing diameter.