For what it is worth:
Voltage step up is generally done with a boost converter, while voltage step down is generally done with a buck converter. Either can be efficient or inefficient according to the design details. Buck converters are much more common and there are some that can offer 90% or 95% efficiency under some circumstances. My slug-o-cuter includes a boost converter so I can hit the slugs with 18V. So, yes the voltage starts at about 18 from the panel, gets bucked down (by the commercial charger module) to really about 13V for the SLA battery, and then gets boosted back up to 18V. Given the desire to include the battery for overnight operation, trying to just keep everything at 18V would be more difficult. The voltage change also facilitates battery charge control.
But none of the above requires fast or slow charging. The solar panel and the boost or buck voltage conversion can provide higher power for faster charging or only lower power for slower charging according to its design.
Buck converters are more common because high voltage is better for sending high power over a distance, while the loads tend to operate from lower voltages. That is why power utilities send out power at many thousands of volts rather than 120 or 240 VAC, although they mainly just use 50/60 Hz transformers to do the voltage conversion.