Brain performance starts degrading due to lack of oxygen at and above 10,000ft, since the body can no longer fully compensate (at least, that's the "official" number). But below FL100 there's the 250kt speed restriction. So, for a fast jet somewhere over the atlantic, descending
There is no speed restriction that far off shore.
2. Speed restrictions of 250 knots do not apply to aircraft operating beyond 12 NM from the coastline within the U.S. Flight Information Region, in offshore Class E airspace below 10,000 feet MSL. However, in airspace underlying a Class B airspace area designated for an airport, or in a VFR corridor designated through such as a Class B airspace area, pilots are expected to comply with the 200 knot speed limit specified in 14 CFR Section 91.117(c). (See 14 CFR Sections 91.117(c) and 91.703.)