  Coronal mass ejections (CMEs) are tightly related to filament eruptions and
usually are their continuation in the upper solar corona. It is common practice
to divide all observed CMEs into fast and slow ones. Fast CMEs usually follow
eruptive events in active regions near big sunspot groups and associated with
major solar flares. Slow CMEs are more related to eruptions of quiescent
prominences located far from active regions. We analyze ten eruptive events
with particular attention to the events on 2013 September 29 and on 2016
January 26, one of which was associated with a fast CME, while another was
followed by a slow CME. We estimated the initial store of free magnetic energy
in the two regions and show the resemblance of pre-eruptive situations. The
difference of late behaviour of the two eruptive prominences is a consequence
of the different structure of magnetic field above the filaments. We estimated
this structure on the basis of potential magnetic field calculations. Analysis
of other eight events confirmed that all fast CMEs originate in regions with
rapidly changing with height value and direction of coronal magnetic field.
