CPU % or efficiency column

[fred]

The CPU % column displays the percentage between the wall clock and the time the project task runs.
100% = the task runs 100% of the time.

CPU

Cpu tasks mostly run close to 100% of the time. But the computer does other things besides BOINC and the value will likely be less than 100%.
And of course using the build in throttle, or better TThrottle, will can cause the percentage to be a lot less.

GPU

Gpu tasks mostly run a lot less than 100% as the Gpu is doing additional work.

Check: "CPU % Long time average"

When this check is enabled BoincTasks will take the whole time period, from the start, to measure the CPU %.
Disabling this check, BoincTasks will take a shorter interval to measure the CPU %. This will better recognize stuck tasks, when the warning is enabled.

Always 100%

BoincTasks versions before 0.40 sometimes only gave 100%, mostly on older BOINC clients.
Go to Extra -> Settings BoincTasks, tab Tasks: Remove the check before "CPU % Long time average".
This will measure the CPU % over a shorter period of time and will enable the automatic 100% detection mode. The last will detect 100% and use a different method to obtain the CPU %.