Heroes in general hold back. But all you need to do is look at any showing where he "lets loose" to know that he isn't going 100% all the time.
Generally it follows the following formula:
Spider-Man randomly happens upon a robbery/kinapping/attack/etc. and attempts to break it up. He may or may not, but it is part of a larger scheme. 1-2 non-100% fights with banter and cheeky dialogue while the main villian steals/kinaps/destroys something. An explosion, person in distress, or plot device stops Spidey from pursuing while the villian gets away. The plan comes together, and Pete must stop it from coming to fruition during the "finale". There's about 1-2 pages of not holding back, which usually results in a pwned and humiliated villian. A page or two of exposition wraps it up as the villian is taken to jail, Spidey takes the innocent boy/girl back to their home, goes home and realizes he missed an important {insert event} with Aunt May or MJ, and has some kind of moralizing statement to make about the story.
...
Scans exist that say he holds back most times. I remember one specifically from the 70's where he's afraid of killing a villian (Mysterio?) so he only uses his wrists to punch rather than swinging with a full arm. But I'm not sure I have any of them scanned into my cpu, and finding them would be a b*tch. DC might have some.
And I can't add a poll to a thread.
...also, there's no argument to be made for Murdock being as fast as Pete. None. And the few fights they've had where Spidey is actually trying, it's over in about a panel. I have no idea what fights he's referring to in the above posts, but there's a couple in my older respect thread where he literally one-shots Matt and DD can't lay a hand on him.