Speed of TrueCrypt whole disk encryption

"I don't have specific numbers, but there will be some drop in performance, albeit a slight one.
There was a blog post that described the general performance of complete disk encryption on a system partition and how that can affect the users perceived performance. It seems to indicate that CPU takes a bigger performance hit than the hard disk:

For me the critical question was what kind of overhead does encrypting your hard drive have on the performance of the system as a whole. To try and measure this I used HDTune to measure drive performance before and after encryption with TrueCrypt. Before encryption I saw an average transfer rate of about 47MB/s with my laptop's SATA drive in AHCI mode. Max was about 59.9MB/s and a burst rate of 85.8MB/s with an average CPU usage of 4.1% throughout the test.HD Performance before Truecrypt
After encryption I saw an average transfer rate of 46.9MB/s, peak of 59.7 MB/s, burst of 62.9MB/s and an average CPU usage of 26.7%. I didn't expect it, but that's where I saw my hit. It makes sense when you think about it - the encryption/decryption of the data generally doesn't result in reading/writing significantly more data (encryption and decryption is done at the block or sector level not at the entire file level) so you don't see substantially reduced disk performance.

Tom's Hardware also has a good article, ""Protect Your Data With Encryption"", that details the performance implications of using TrueCrypt on a complete volume.
Also, see Scott Gu's blog post regarding the speed of the physical hard disk (i.e. the RPM) (Tip/Trick: Hard Drive Speed and Visual Studio Performance) which can make a big difference to Visual Studio's performance whether encryption is employed or not.
There's another post here: What is the Performance Impact of System Encryption With TrueCrypt"

I did a number of tests compiling a large project (takes about 10 minutes to compile) on a Windows 7 desktop. There was absolutely no difference in my build times before and after TrueCrypt-ing the hard drive (using AES).

for what its worth, I have a 4 core (8 threads) machine and use a 1TB partitioned disk, 500GB encrypted and 500 GB non encrypted. Compiling a project on the encrypted partition takes 40 minutes or so and the CPU is idle for most that. Compiling on the non encrypted disk takes around 6 minutes. That's with multiprocessing compiling enabled in VS2010. I think the encryption becomes a severe bottle neck when you have many cores.

"My employer install PGP whole disk encryption on my Lenovo W500 and the subjective perfomance hit is a 30% to 50%. Everything is much, much slower. Most importantly start up and all program starts are delayed remarkably. Next issue: is disk defragmentation still working - cannot find any information and the performance hit may indicate there is some kind of issue.
Recommend what I do at home: Encrypt data only, don't use hybernation, and ask yourself how important are your data that someone makes the effort to analyze your pagefile."

I'd expect performance to drop heavily when writing but not as seriously when reading. You could monitor your Visual Studio disk and memory usage, compare it with what you can find in the web, and use delayed disk writes whenever possible.