Hello, and welcome! I'm glad you've stopped lurking and have joined the party!
Since you are using pppoe(4), I wonder if you are experiencing degradation due to MTU settings. If I recall correctly, PPPoE had an upper MTU bound of 1492 bytes until RFC 4638 was instituted at OpenBSD 5.1. If your Linksys equipment is not RFC 4638 capable, or is configured to a lower MTU, perhaps MTU needs to be adjusted. The
pppoe(4) man page shows 1492 being used in its published configuration example.
I also wonder if the
pool(9) limitation you instituted for frags is pouring oil on flames, if you happen to have fragmentation caused by mismatched MTUs.
But I'm not a pppoe(4) user, so I don't write from any operational knowledge.