How is the correct air speed for fuel combustion obtained at the inlet of the combustor?
Combustors usually feature recirculation zones in which the combustion takes place either completely or which at least anchor the flame. This can be done by several designs


Sudden jump in cross-section in combination with vorticity
Flame-holder bodies
Central displacers
Vorticity brake-down

The blade channel of a compressor is actually divergent. Deceleration of a flow goes along with an increase of the static pressure (see diffusors). Additionally, a compressor transfers work into the fluid, thus further increasing the fluid density. The divergent cross-section of the annular channel between hub and casing is designed to keep the axial velocity somehow level. High axial (or more correctly meridional) velocity levels cause higher friction losses. Low velocities reduce the transferable power of a compressor stage, thus requiring more stages.