By default Glimpse is enabled on local environments only and won't run on production until you add this into your web.config:
<glimpse defaultRuntimePolicy="On" endpointBaseUri="~/Glimpse.axd">
<add type="Glimpse.AspNet.Policy.LocalPolicy, Glimpse.AspNet" />
I tested it myself when after the first Live rollout we accidentally left Glimpse installed (but without ignoring the Local policy listed above) and Glimpse didn't work without it. And I haven't noticed any negative performance impact (it should be, of course, as Michal mentioned, one small check is performed on the code but it looks like an extra small "if" statement if Glimpse is disabled on Live site).