In aphorism 42 of Minima Moralia, Adorno writes that
Freedom of thought. – The suppression of philosophy by science has
led, as is commonly known, to a separation of the two elements whose
unity, according to Hegel, comprises the life of philosophy:
reflection and speculation... Speculation is hit hard from the outset by the separation from reflection. It either degrades into the compliant parroting of traditional philosophical schemes or degenerates, in its distance from facts which have been rendered blind, into the babble of a non-binding private world view.
i.e. speculation these days is either a repetition of convention or a circumvention of the facts.
In this aphorism he then says that speculation also now appears (deformed) in the sciences, e.g. psychoanalytic free association, in which the patient is absolved of the need to labour on (think through) his thoughts.
This page says that speculation is the process whereby contradictions, revealed via dialectics, are resolved into higher unities.
Clearly, Adorno thought that there was a telling need for this speculation to be negative, which would do justice to Marx's (since crippled) judgment that philosophy had only interpreted the world. You can see that in the intro of Negative Dialectics
Speculative Moment 27-29. Even after renouncing idealism, it [philosophy] cannot dispense with
speculation, albeit in a wider sense than Hegel’s all too positive one... Positivists are quick to write off Marxist materialism,
which is one of objective laws of essence, which by no means proceed from immediate
data or sets of axioms, as speculation.