did you solve your problem on slow bonding link?
i have the same issu.
I've tried numerous types of configurations without any luck.
All types of VPN and tunnels that Mikrotik supports.
3 different routers in case there was a problem with the hardware.
7 different modems.
3 different ISPs
2 different datacenters on the other end.
different USB controllers for each modem in case the usb hub or the modems cannot simultaneously fully use the same usb channel.
All sorts of settings on the bonding interface.
I even used separate routers (Draytek Vigor) for the modems and bonded their ethernets on a 3rd Mikrotik router.
And I always get the same result:
When 2 or more modems are uploading at the same time, the upload speed suffers.
Each modem individually can upload up to 1mbit.
But together the aggregated speed cannot exceed 1mbit.
So with 2 modems their upload speed when uploading together is maximum ~500k
With 3 modems this goes down to ~330k
And so on...
I think it may be RF interference between the modems when they all transmit causing this behavior.
I haven't had the chance to confirm that, but after everything I tried I think it's the only thing left out.
But what's weird about that is that the total aggregated speed is always 1mbit.
If it was RF interference wouldn't the results vary?