awk '/Speed:/ {print $3}' screenlog.0

pilot6@Pilot6:~$ echo "Total Speed: 118.604 Mh/s, Total Shares: 5...." | awk '/Speed:/ {print $3}'
118.604

... |tail -n3 | grep -Pzo '(?<=Total Speed: ).*?(?=Mh/s)' infile.txt

118.604
118.604
118.604