What exactly is a "fast" oven?

The idea of a "fast" oven comes from the time when wood-burning stoves without temperature gauges were the most common ones in kitchens. A fast oven is anywhere from 400-425° F.

Conversion to Fahrenheit

Very slow (very low) oven: 300-325° F.
Slow (low) oven: 325-350° F.
Moderate (medium) oven: 350-375° F.
Fast/quick (high) oven: 375-400° F.
Very fast/very quick (very high) oven: 400-425° F.

Various "modern" cookbooks that attempt to explain these terms will often give different temperatures, but the degree of difference isn't usually that great (usually about 25 degrees in either direction); what was a fast oven to one cook may only have been a moderate oven to another.

A "pastry," "bread," or "bread-baking" oven generally refers to a temperature range of 360-380° F.

(Rounded) Conversion to Celsius

Very slow (very low) oven: 150-160/170° C.
Slow (low) oven: 160/170-180° C.
Moderate (medium) oven: 180-190° C.
Fast/quick (high) oven: 190-200° C.
Very fast/very quick (very high) oven: 200-220° C.

Conversion to Gas Marks

Very slow (very low) oven: 2-3
Slow (low) oven: 3-4
Moderate (medium) oven: 4-5
Fast/quick (high) oven: 5-6
Very fast/very quick (very high) oven: 6-7

Oven To Campfire Temperature Conversion Chart
(For Use with Dutch Ovens and Charcoal)

Slow (low) oven: 6-8 briquettes below/ 12-16 on lid
Moderate (medium) oven: 8-10 briquettes below/ 16-18 on lid
Fast/quick (high) oven: 10-12 briquettes below/ 18-24 on lid

It varies.

• 'On temp' fast. Quickly reaches the desired temperature
• 'Cooks fast'. A combination of forced air, microwave, light waves, grilling: technologies to get the food ready faster.