The mechanism for disk writes in pfsense for nanobsd was changed in 2.2.3 or somewhere around there. Net effect was exactly what you are describing but it should have been fixed in 2.2.4 if memory serves. Fact that you are using 2.2.6 I would not think you should be seeing that behavior. Try a different USB key. I have the same chipset (BLKD2500CCE Intel Desktop Board D2500CC) but I'm using a USB to SATA flash card adapter SYBA SD-ADA50024 and a WinTec FileMate 8GB 3FMCF8GBS-R compact flash card. My backups are several Kingston DataTraveler USB keys (if the sata to USB dies) but I haven't used those enough (other than just booting to confirm functionality) enough to know if they have the slow down you describe or not.
https://doc.pfsense.org/index.php/2.2.4_New_Features_and_Changes#Security.2FErrata_Notices
The forcesync patch for https://redmine.pfsense.org/issues/2401 is still considered harmful to the filesystem and has been kept out. As such, there may be some noticeable slowness with NanoBSD on certain slower disks, especially CF cards and to a lesser extent, SD cards. If this is a problem, the filesystem may be kept read-write on a permanent basis using the option on Diagnostics > NanoBSD. With the other above changes, risk is minimal. We advise replacing the affected CF/SD media by a new, faster card as soon as possible. https://redmine.pfsense.org/issues/4814