To echo some other comments, we may be sidetracking off the OP's question, which is really quite a high level one.
Also agree it depends on what you mean by "performance". Perhaps a starting point maybe to have a look at the theory behind "Flight Envelopes". This was my first introduction to performance theory back in my teens when started reading some discussion from Chuck Yeager I got a hold of on why aircraft "perform" differently at different altitudes
en.wikipedia.org/wiki/Flight_envelope
In terms of ability to accelerate, climb or maneuver, generally this is better at lower altitude (and worse at higher altitude) due to higher lift and higher excess power (Generally, although that can depend on the powerplant and airframe combination in question). These are probably the key "performance" characteristics to keep in mind when looking to fly the plane safely. Certainly, this has been topical in the spate of high-altitude stalls/incidents.
Now, in terms of speed, I tend to feel true "performance" is true airspeed. Yes IAS can be different, but on a discussion on aircraft performance...somewhat irrelevant unless we want to talk about a question separate from that the OP asked.
Maximum speed depends on the airframe and powerplant in question. Some planes are designed they will achieve this at near ground level, others will achieve it at higher altitudes.