Extremely slow Performance of MXF OP1a Files in Premiere Productions

Please tell us your system specs: OS version, Premiere version, amount of RAM, Hardware specs including graphics card and what are pixel dimensions of the mxf files?

Have you tried a proxy workflow?  Not the easiest thing to wrap your head around, but works great once you do.  If I remember correctly, some sony cameras shoot with 8 channels of audio which can be a serious impediment to generating proxies.  I was never able to make it work so I've always transcoded to prores with 2 channels of audio and then generated proxies from there.