I guess what you need to evaluate is why you need to encrypt the machine.
The reason may as well help us to determine what's best for you. Encryption these days, especially when you are talking about whole disk encryption, is basically only useful to prevent one attack - which is, somebody getting your machine while it was powered off, and you want to prevent them from getting the data inside.
For most other purpose, you better invest in
(1) network access control, which includes, but not limited to, wireless security and use of encrypted protocol (e.g. ssh as opposed to telnet, https as opposed to http), use of VPN when in public networks
(2) file-based, or container-based security, that is, something like truecrypt -- this is the case when you do have something to hide (illegal, secret or otherwise), and because this is not whole disk encryption you won't have to mount it all the time.
(3) physical security - it's old saying that physical access equals root. while this may not be as true nowadays, there is still some weight.
For your question -
- whether it will decrease performance if the drives are logically separated - Not usually. Bitlocker volume are only encrypted/decrypted when there is access, and encrypting nowadays is a very simple matter, and won't tax your CPU much unless your secrets are big, big streams of video or images for that matter
- read/writes will not tax the harddrive more than 1.1x (with overheads) it is when it was not encrypted. CPU load is negligible these days if R/W is not significant on data drive during gaming.