Nope.
The ability to have Low Priority interrupts depends on the hardware, and 14-bit cores don't have it.

I would suspect that you are using the RX interrupt to start reading a long string of bytes using HSERIN in the handler.
If so, then it's sitting in the handler doing nothing while waiting for data to come in.

For best speed, an RX interrupt should only retrieve 1 byte at a time, then exit the handler.
It only takes a few uS that way.