Default [Q] Any workaround for SLOW Google Voice app SMS?

I use the Google Voice app to send SMS because I want to send SMS from my GV number, not my VZN number. Correct me if I am wrong, but it is my understanding that is the only way that works currently. I know Cyanogenmod has some extra integration with Google Voice but I have a Galaxy S 5 so CM is not an option (locked bootloader).

My problem with the Google Voice app is that I get delays of 5+ minutes sending SMS where it goes into "Queued" and "Sending" for long periods of times, then sometimes it has to fail and retry to send. Eventually they always go through but it might take 10+ minutes.

I'm not sure what causes the delay or if there is a workaround. I've seen stuff online about deleting old conversation threads (which sounds dubious to me - why would that cause a delay?) but I tried it and it made no difference. Any suggestions as to workarounds for this issue? Thanks.