Heat soak occurs when the intercooler can't shed the heat that it removes from the compressed air of the turbo. On a hot day, the intercooler can, like a sponge, become "soaked" with heat and lose its effectiveness.
Heat soak is one of the major reasons that turbocharged cars tend to run slower when the weather is warm.
Common solutions to improve heat soak is the use of a higher-capacity intercooler, or one that's mounted more in-line with the air flow, as in front-mount intercoolers.