Johnson has terrorised England in the current Ashes series and took 7-40, including five wickets in 18 balls, in the second Test in Adelaide on Saturday.
The left-armer repeatedly broke the 150km/h mark but McDermott believes Johnson can get quicker.
"It was as good as any spell you will see in world cricket," McDermott told ABC radio on Sunday.
"Great pace, good control ... he's in a really good space and he's really comfortable within his own skin, that is the biggest thing about anyone I suppose, once you're relaxed and comfortable.
"I reckon he can bowl faster."
Johnson rose to 10th on Australia's list of Test wicket-takers with his feats.
"We have got a really well balanced attack. Mitchell is obviously about 10k's quicker than everyone else at the minute, so he is really the enforcer," McDermott said.
The aggressive nature of Johnson and his fellow fast bowlers was a key factor, he said.
"It's really important that we're trying to perform as a group and be really aggressive," McDermott said.
"We have just got to make sure that we stick to that aggression.
"And blokes (batting) at seven or eight, you have got to give them a bit of a hard time and we're doing that - not only to their tailenders but to their top order as well."