The CSS voice-rate property specifies the speaking rate in words per minute. Set the rate with the following values
voice-rate: [normal | x-slow | slow | medium | fast | x-fast]
Above, if you want slow voice rate, then set slow; for slower, set x-slow.
Set faster voice rate: p { voice-rate: x-fast; }
Set voice rate in percentage
p { voice-rate: 40%; }