Can a "speed" be "fast"/"slow"?

show that 'fast speed' and 'slow speed' are actually used, but are not as idiomatic as the expressions 'high speed' and 'low speed'. Many would indeed say that 'fast speed' and 'slow speed', invoking speed twice, are tautologous and therefore stylistically poor.

However, since speed is almost synonymous with setting when considering say kitchen devices (and compare with a ten-speed racing bike), there is an increasing usage with examples such as that given in Collins CoBuild (the second below):

slow [adjective]:

Something that is slow moves, happens, or is done without much speed.

• The traffic is heavy and slow.
• Electric whisks should be used on a slow speed.

But 'the car's speed is fast' is certainly better avoided.