Encrypt or Secure a file

Question

Hi

I wrote my own database class and it works great and fast... but the moment I added encryption to my tables loading my tables started to take LONG and with a bit of testing I discovered it was the decryption taking forever...

I want to secure either the file or encrypt the table(s) so only my class can read it. I dont know much about encryption and im using RijndaelManaged

is there anyway I can speed it up or should I try this from a different angle ?

I was thinking of writing my own basic encryption methods just like a "Decoder Ring" but I keep thinking there has to be a better way...

Any Input appreciated.
Thx
Rousseau