Matthew McGiffen has a few tests on using Transparent Data Encryption:
By the time it had been executed 5 times (with the memory flushed between each execution) each query read about 600,000 pages sized at 8kb each – just under 5GB. If it took 50 seconds on the decryption of those pages, then each page took about 1 twelfth of a milli-second to decrypt – or alternatively, TDE decrypted about 12 pages per millisecond. Or in terms of disk size, 100MB per second. These were tests on a server with magnetic spinning disks (not SSDs) and you can see from the above figures, the straight disk access took about 40 seconds on its own.
When TDE doesn’t read from disk it doesn’t add any overhead, but how do we quantify what the overhead to queries is when it does have to access the disk?
Matthew has some good advice here, and I’d be willing to say that his experience is within the norm for TDE and doesn’t directly contradict general guidelines by enough to shift priors.