Impact of Bitlocker on performance of SSD
See a similar superuser question here.
Impact of Bitlocker on lifespan of SSD
Microsoft's answer to the question:
"Is Bitlocker’s encryption process optimized to work on SSDs?"
Yes, on NTFS. When Bitlocker is first configured on a partition, the entire partition is read, encrypted and written back out. As this
is done, the NTFS file system will issue Trim commands to help the SSD
optimise its behaviour.
From a similar superuser question:
If the disk controller does not use compression, then encryption will
not change anything. If the controller uses compression then
encryption will probably reduce the lifespan of the disk (compared to
an identical disk where encryption is not used).
From "random" blog posts: *See conclusion.
(For Software Based Encryption eg Bitlocker):
Even when you change a single bit in file, due to the re-encryption of
the file, the whole file will be written back to the SSD and not only
the changed block of data. This will incur additional wear-and-tear of
the SSD, reducing the performance exponentially.
You can prevent this additional reduction of performance by using Self Encrypting Drives (SED). A SED will encrypt data in the hardware of the drive. So you win on drive performance two times:
Encryption is done in real-time (e.g. when SW FDE is applied it will reduce immediately 40-50% of SSD read and write performance)
inefficient use of the SSD is prevented by optimization routines in the controller of the SED.
SSDs get a lot of their performance and longevity from doing extremely
smart reads and writes at the block level and by compressing the data
intelligently. When presented with an encrypted volume, the SSD can
make no determinations about the data and has to read and write entire
blocks at a time. There's typically a pretty big performance penalty
as well, and it wears the drive a lot faster.
I believe that the superuser answer I linked to is more detailed, so you'd need to find out whether your SSD encrypts the data to save space, then make your decision.
As Ramhound said, "a random blog post by a random person isn't the best source of information", I think that the information provided is slightly inaccurate as it assumes that all the data is rewritten to encrypt it, while in reality there should be no need to perform extra reads or writes because the data is encrypted before being written to the platters, and decrypted before being sent to the process that performed the read.
Overall my opinion is that the security outweighs the lifespan, as you're not likely to hit the write limit on modern SSDs in a reasonable number of years.
Sources: Here, here, here and here.