A slower speaking rate of human or synthetic speech is often requested by for example language learners or people with aphasia or dementia. Slow speech produced by human speakers typically contain a larger number of pauses, and both pauses and speech have longer segment durations than speech produced at a standard or fast speaking rate. This paper presents several methods of prolonging speech. Two speech chunks of about 30 seconds each, read by a professional voice talent at a very slow speaking rate, were used as reference. Seven pairs of stimuli containing the same word sequences were produced, one by the same professional, reading at her standard speaking rate and six by a moderately slow synthetic voice trained on the same human voice. Different combinations of pause insertions and stretching were used to match the total length of the corresponding reference stimulus. Stretching was applied in different proportions to speech and non-speech, and pauses were inserted at punctuations, at certain phrase boundaries, between each word, or by copying the pause locations of the reference reading. 128 crowdsourced listeners evaluated the 16 stimuli. The results show that all manipulated readings are less consistent with expectations of slow speech than the reference, but that the synthesised readings are comparable to stretched human speech. Key factors are the relation between speech and silence and the duration of talkspurts.