Just a short notice from my side (as I haven't used Gladinet yet):
Using BoxCryptor increases the number of file operations (like get file information, open a file, get directory listing, etc.). For example a task which would involve "normally" 5 file operations in plaintext may require 10 (or even more) file operations when performed through BoxCryptor. One reason for this is filename encryption where we need different access patterns.
This is not a problem at all if the BoxCryptor Folder is located on your local hard disk because access and response times are so fast (1-50 ms) that it makes no difference if 5 file operations are executed or 20.
However, if the BoxCryptor Folder is located on a remote network location, access times are greatly higher (> 100ms) and also the overhead to execute a file operation is a lot bigger. In this case additional file operations can greatly reduce the performance.
That's why we recommend to use a cloud storage provider with a true syncing process where the BoxCryptor Folder can be located on a local hard disk - as most of our users do. I assume this is not the case when you use Gladinet (which then stores the data in Google Drive). It would be better to directly use the Google Drive sync client (at least for the first data import).
I don't think that encryption causes your slower performance (we have ~ 40-50 MB/sec raw encryption speed on a Core i5/7 processor). I think it's rather the increase in the number of file operations in combination with the high latency over network which causes this performance troubles - so the bottleneck is not your cpu or bandwith, but actually the latency.
PS: Encryption could be a reason for slower transfer speeds if Gladinet uses compression before the transfer (which many are doing) and you transfer data with a good compression ratio (e.g. text files). Ciphertext is almost "uncompressable" due to its random nature, so if the plaintext could have a 50% compression ratio, transfer times would double for ciphertext. But I don't think this is the reason in your case.