Openvpn: Which encryption is the least CPU intensive?

Question

openvpn

I have an mr3020 router with an lte whistle, due to some objective reasons (the operator's unreal greed) I let traffic through openvpn to my server in the DC. I observe a decrease in speed by 4-5 times.
The server part is configured as tls, with a 2048-bit encryption key.
I want to change the settings to unload the cpu of the router and increase the speed.
Advise the openvpn mode, in which there is at least some kind of weak encryption (the main thing is that there is no bare traffic) and the cpu of the router is least loaded?
comp-lzo is not available, only tap

Answer 1

I tried the following settings:
on both
cipher BF-CBC
tun-mtu 1500
fragment 1300
mssfix
fast-io
verb 0
on the server
push "sndbuf 393216"
push "rcvbuf 393216"
sndbuf 393216
rcvbuf 393216
The speed almost doubled..