Slowdown when accessing two MIDI devices simultaneously

[Laurence Levin]

I have a pretty powerful older computer with an i5 quad core chip and lots of memory.

I have no problem

playing and recording Pianoteq 6 softsynth using my Roland keyboard as a MIDI controller +  playback of pre-recorded audio.

I DO HAVE A PROBLEM: Playing  or recording Pianoteq softsynth using my Roland keyboard as a MIDI controller + playback of an eight track MIDI playing through my Yamaha keyboard.

 I get a slowdown. A few times, I got an error message in Cakewalk by Bandlab that there wasn't enough memory for my Roland MIDI driver. Huh??? I thought a MIDI driver was very light on resources! Roland doesn't make a proprietary MIDI driver, btw. 

So I have to convert my MIDI project to audio before I can play along with my softsynth. 

Is there anything I can do in settings? 

I'll be happy to clarify, if I'm not making sense!!