micropdp11 and virtual memory

[Douglas Taylor via cctalk]

I've been compiling and running some Fortran programs on my 
microPDP11/53 computer and have questions about Fortran VIRTUAL arrays.

Details: KDJ11-D/S cpu with 0.5 MB of ram, additional 1MB of Qbus ram.  
I'm running RT11 V5.7 ZM and using the F77XM compiler, version 5 (I think).

What I am seeing is that when I use VIRTUAL arrays instead of the usual 
memory below 64K the run times are dramatically slower.  Is this normal?

Could it be something else?  The other memory board?